Hydraulic brake system with controlled boost

ABSTRACT

The brake system includes a pedal, a pedal simulator, and a brake pedal unit. The brake pedal unit includes an input piston connected to the brake pedal for operating the pedal simulator during a normal braking mode. The brake pedal unit further includes first and second output pistons actuated by the input piston during a manual push through mode such that the first output piston generates brake actuating pressure at a first output of the brake pedal unit, and the second output piston generates actuating pressure at a second output of the brake pedal unit. The brake system further includes a hydraulic pressure source for supplying fluid at a controlled boost pressure, and a hydraulic control unit adapted to be hydraulically connected to the brake pedal unit and the hydraulic pressure source. The hydraulic control unit includes a slip control valve arrangement and a switching base brake valve arrangement for switching the brake system between the normal braking mode, wherein boost pressure from the hydraulic pressure source is supplied to first and second vehicle brakes, and the manual push through mode wherein the brake actuating pressure from the first and second output pistons is supplied to the first and second vehicle brakes, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/406,730, filed Oct. 26, 2010, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

This invention relates in general to vehicle braking systems. Vehiclesare commonly slowed and stopped with hydraulic brake systems. Thesesystems vary in complexity but a base brake system typically includes abrake pedal, a tandem master cylinder, fluid conduits arranged in twosimilar but separate brake circuits, and wheel brakes in each circuit.The driver of the vehicle operates a brake pedal which is connected tothe master cylinder. When the brake pedal is depressed, the mastercylinder generates hydraulic forces in both brake circuits bypressurizing brake fluid. The pressurized fluid travels through thefluid conduit in both circuits to actuate brake cylinders at the wheelsto slow the vehicle.

Base brake systems typically use a brake booster which provides a forceto the master cylinder which assists the pedal force created by thedriver. The booster can be vacuum or hydraulically operated. A typicalhydraulic booster senses the movement of the brake pedal and generatespressurized fluid which is introduced into the master cylinder. Thefluid from the booster assists the pedal force acting on the pistons ofthe master cylinder which generate pressurized fluid in the conduit influid communication with the wheel brakes. Thus, the pressures generatedby the master cylinder are increased. Hydraulic boosters are commonlylocated adjacent the master cylinder piston and use a boost valve tocontrol the pressurized fluid applied to the booster.

Braking a vehicle in a controlled manner under adverse conditionsrequires precise application of the brakes by the driver. Under theseconditions, a driver can easily apply excessive braking pressure thuscausing one or more wheels to lock, resulting in excessive slippagebetween the wheel and road surface. Such wheel lock-up conditions canlead to greater stopping distances and possible loss of directionalcontrol.

Advances in braking technology have led to the introduction of Anti-lockBraking Systems (ABS). An ABS system monitors wheel rotational behaviorand selectively applies and relieves brake pressure in the correspondingwheel brakes in order to maintain the wheel speed within a selected sliprange to achieve maximum braking force. While such systems are typicallyadapted to control the braking of each braked wheel of the vehicle, somesystems have been developed for controlling the braking of only aportion of the plurality of braked wheels.

Electronically controlled ABS valves, comprising apply valves and dumpvalves, are located between the master cylinder and the wheel brakes.The ABS valves regulate the pressure between the master cylinder and thewheel brakes. Typically, when activated, these ABS valves operate inthree pressure control modes: pressure apply, pressure dump and pressurehold. The apply valves allow pressurized brake fluid into respectiveones of the wheel brakes to increase pressure during the apply mode, andthe dump valves relieve brake fluid from their associated wheel brakesduring the dump mode. Wheel brake pressure is held constant during thehold mode by closing both the apply valves and the dump valves.

To achieve maximum braking forces while maintaining vehicle stability,it is desirable to achieve optimum slip levels at the wheels of both thefront and rear axles. During vehicle deceleration different brakingforces are required at the front and rear axles to reach the desiredslip levels. Therefore, the brake pressures should be proportionedbetween the front and rear brakes to achieve the highest braking forcesat each axle. ABS systems with such ability, known as Dynamic RearProportioning (DRP) systems, use the ABS valves to separately controlthe braking pressures on the front and rear wheels to dynamicallyachieve optimum braking performance at the front and rear axles underthe then current conditions.

A further development in braking technology has led to the introductionof Traction Control (TC) systems. Typically, valves have been added toexisting ABS systems to provide a brake system which controls wheelspeed during acceleration. Excessive wheel speed during vehicleacceleration leads to wheel slippage and a loss of traction. Anelectronic control system senses this condition and automaticallyapplies braking pressure to the wheel cylinders of the slipping wheel toreduce the slippage and increase the traction available. In order toachieve optimal vehicle acceleration, pressurized brake fluid is madeavailable to the wheel cylinders even if the master cylinder is notactuated by the driver.

During vehicle motion such as cornering, dynamic forces are generatedwhich can reduce vehicle stability. A Vehicle Stability Control (VSC)brake system improves the stability of the vehicle by counteractingthese forces through selective brake actuation. These forces and othervehicle parameters are detected by sensors which signal an electroniccontrol unit. The electronic control unit automatically operatespressure control devices to regulate the amount of hydraulic pressureapplied to specific individual wheel brakes. In order to achieve optimalvehicle stability, braking pressures greater than the master cylinderpressure must quickly be available at all times.

Brake systems may also be used for regenerative braking to recaptureenergy. An electromagnetic force of an electric motor/generator is usedin regenerative braking for providing a portion of the braking torque tothe vehicle to meet the braking needs of the vehicle. A control modulein the brake system communicates with a powertrain control module toprovide coordinated braking during regenerative braking as well asbraking fox wheel lock and skid conditions. For example, as the operatorof the vehicle begins to brake during regenerative braking,electromagnet energy of the motor/generator will be used to applybraking torque (i.e., electromagnetic resistance for providing torque tothe powertrain) to the vehicle. If it is determined that there is nolonger a sufficient amount of storage means to store energy recoveredfrom the regenerative braking or if the regenerative braking cannot meetthe demands of the operator, hydraulic braking will be activated tocomplete all or part of the braking action demanded by the operator.Preferably, the hydraulic braking operates in a regenerative brakeblending manner so that the blending is effectively and unnoticeablypicked up where the electromagnetic braking left off. It is desired thatthe vehicle movement should have a smooth transitional change to thehydraulic braking such that the changeover goes unnoticed by the driverof the vehicle.

SUMMARY OF THE INVENTION

This invention relates to vehicular brake systems and, in particular, toa brake system having controlled hydraulic boost. The brake systemincludes a pedal, a pedal simulator, and a brake pedal unit. The brakepedal unit includes an input piston connected to the brake pedal foroperating the pedal simulator during a normal braking mode. The brakepedal unit further includes first and second output pistons actuated bythe input piston during a manual push through mode such that the firstoutput piston generates brake actuating pressure at a first output ofthe brake pedal unit, and the second output piston generates actuatingpressure at a second output of the brake pedal unit. The brake systemfurther includes a hydraulic pressure source for supplying fluid at acontrolled boost pressure, and a hydraulic control unit adapted to behydraulically connected to the brake pedal unit and the hydraulicpressure source. The hydraulic control unit includes a slip controlvalve arrangement and a switching base brake valve arrangement forswitching the brake system between the normal braking mode, whereinboost pressure from the hydraulic pressure source is supplied to firstand second vehicle brakes, and the manual push through mode wherein thebrake actuating pressure from the first and second output pistons issupplied to the first and second vehicle brakes, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a first embodiment of a brakesystem.

FIG. 2 is an enlarged schematic sectional view of a brake pedal unit ofthe brake system of FIG. 1 shown in its rest position.

FIG. 3 is an enlarged schematic sectional view of the brake pedal unitof FIG. 2 shown in its boost apply position.

FIG. 4 is an enlarged schematic sectional view of the brake pedal unitof FIG. 2 shown in its manual apply position.

FIG. 5 is a graphical representation of a possible pedal force andsimulation pressure vs. pedal travel for the brake system of FIG. 1.

FIG. 6 is a schematic illustration of a second embodiment of a brakesystem.

FIG. 7 is a schematic illustration of a third embodiment of a brakesystem.

FIG. 8 is a schematic illustration of a fourth embodiment of a brakesystem.

FIG. 9 is an enlarged schematic sectional view of a master cylinderassembly of the brake system of FIG. 8 shown in its rest position.

FIG. 10 is a schematic illustration of the brake system of FIG. 8 shownin a normal boost apply position.

FIG. 11 is a schematic illustration of the brake system of FIG. 8 shownin a failed condition.

FIG. 12 is a cross-sectional view of an embodiment of the mastercylinder assembly of FIG. 9.

FIG. 13 is a graphical representation of a possible pedal force andsimulation pressure vs. pedal travel for the brake system of FIG. 8.

FIG. 14 is a cross-sectional view of an embodiment of a medium pressureaccumulator.

FIG. 15 is a cross-sectional view of an embodiment of a base brakevalve.

FIG. 16 is a cross-sectional view of an embodiment of a simulationvalve.

FIG. 17 is an enlarged schematic sectional view of a portion of analternate embodiment of a brake pedal unit shown in its rest position.

FIG. 18 is an enlarged cross-sectional view of a portion of the brakepedal unit of FIG. 17 illustrating the sloped frustoconical surfaces onan input piston.

FIG. 19 is a schematic illustration of an alternate embodiment of abrake system.

FIG. 20 is a schematic illustration of an alternate embodiment of abrake system.

FIG. 21 is an enlarged schematic sectional view of the brake pedal unitof FIG. 20.

FIG. 22 is a schematic illustration of an alternate embodiment of abrake system.

FIG. 23 is a schematic sectional view of an alternate embodiment of abrake pedal unit.

FIG. 24 is a schematic illustration of an alternate embodiment of abrake system.

FIG. 25 is a schematic illustration of an alternate embodiment of abrake system.

FIG. 26 is a cross-sectional view of an alternate embodiment of a brakepedal unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is schematically illustrated inFIG. 1 a first embodiment of a vehicle brake system, indicated generallyat 10. The brake system 10 is a hydraulic boost braking system in whichboosted fluid pressure is utilized to apply braking forces for the brakesystem 10. The brake system 10 may suitably be used on a ground vehiclesuch as an automotive vehicle having four wheels with a wheel brakeassociated with each wheel. Furthermore, the brake system 10 can beprovided with other braking functions such as anti-lock braking (ABS)and other slip control features to effectively brake the vehicle, aswill be discussed below.

The brake system 10 generally includes a hydraulic control unit,indicated by broken lines 12, and a power pack assembly, indicated bybroken lines 14. The components of the hydraulic control unit 12 may behoused together in a single unit or block. The components of the powerpack assembly 14 may also be housed in a single unit or block. Asschematically shown, the power pack assembly 14 is located remotely fromthe hydraulic control unit 12 and conduits or hydraulic lineshydraulically couple the hydraulic control unit 12 with the power packassembly 14. Alternatively, the hydraulic control unit 12 and the brakemodule 14 may be housed in a single unit. It should also be understoodthat the grouping of components as illustrated in FIG. 1 is not intendedto be limiting and any number of components of the hydraulic controlunit 12 and the power pack assembly 14 may be housed together orseparately.

The hydraulic control unit 12 cooperatively acts with the power packassembly 14 for actuating wheel brakes 16 a, 16 b, 16 c, and 16 d. Thewheel brakes 16 a, 16 b, 16 c, and 16 d can be any suitable wheel brakestructure operated by the application of pressurized brake fluid. Thewheel brake 16 a, 16 b, 16 c, and 16 d may include, for example, a brakecaliper mounted on the vehicle to engage a frictional element (such as abrake disc) that rotates with a vehicle wheel to effect braking of theassociated vehicle wheel. The wheel brakes 16 a, 16 b, 16 c, and 16 dcan be associated with any combination of front and rear wheels of thevehicle in which the brake system 10 is installed. For example, wheelbrakes 16 a and 16 b may be associated with the front wheels and wheelbrakes 16 c and 16 d may be associated with rear wheels. For adiagonally split brake system, the wheel brakes 16 a and 16 c may beassociated with one side of the vehicle, and the wheel brakes 16 b and16 c may be associated with the other side of the vehicle.

The hydraulic control unit 12 includes a fluid reservoir 18 for storingand holding hydraulic fluid for the brake system 10. The fluid withinthe reservoir 18 may be held generally at atmospheric pressure or canstore the fluid at other pressures if so desired. The hydraulic controlunit 12 also includes a brake pedal unit (BPU), indicated generally at20. The brake pedal unit 20 is also schematically shown in enlargeddetail in FIGS. 2-4. It should be understood that the structural detailsof the components of the brake pedal unit 20 illustrate only one exampleof a brake pedal unit 20. The brake pedal unit 20 could be configureddifferently having different components than that shown in FIGS. 2-4.

The brake pedal unit 20 includes a housing 24 having various boresformed in for slidably receiving various cylindrical pistons and othercomponents therein. In the embodiment illustrated in FIGS. 2-4, thehousing 24 is formed from a first portion 24 a and a second portion 24b. The first and second portions 24 a and 24 b may be coupled togetherby any suitable manner, such as by mounting bolts. Alternatively, thehousing 24 may be formed as a single unit or include three or moreseparately formed portions coupled together. The housing 24 generallyincludes a first bore 26, a second bore 28, a third bore 30, and afourth bore 32. The first and second bores 26 and 28 are formed in thefirst portion 24 a of the housing 24 and are axially aligned with oneanother. The second bore 28 has a larger diameter than the first bore26. The third and fourth bores 30 and 32 are formed in the secondportion 24 b and are in communication with the larger diameter secondbore 28. The third bore 30 is located laterally (or above when viewingFIGS. 1 and 2) with respect to the fourth bore 32. The use of twohousing portions 24 a and 24 b provides for convenient installation ofvarious components within the bores 26, 28, 30, and 32.

The brake pedal unit 20 further includes an input piston 34, anintermediate piston 36, a first output piston 38, and a second outputpiston 40. The input piston 34 is slidably disposed in the first bore26. The intermediate piston 36 is slidably disposed in the second bore28. The first output piston 38 is slidably disposed in the third bore30. The second output piston 40 is slidably disposed in the fourth bore32.

A brake pedal, indicated schematically at 42 in FIG. 1, is coupled to afirst end 44 of the input piston 34 via an input rod 45 and a coupler46. The coupler 46 is threadably engaged with the first end 44 of theinput piston 34 and includes a socket 48 for receiving a spherical end50 of the input rod 44. Alternatively, the input rod 45 could be coupleddirectly to the input piston 34 without having the coupler 46. The inputpiston 34 includes an enlarged second end 52 that defines a shoulder 54.In the rest position shown in FIG. 2, the shoulder 54 of the inputpiston engages with a shoulder 56 formed between the first and secondbores 26 and 28 of the housing 24. An outer cylindrical surface 57 ofthe input piston 34 is engaged with lip seals 58 and 60 mounted ingrooves formed in the housing 24. The outer cylindrical surface 57 maybe continuous along its length or it may be stepped, as shown in FIG. 2,having two or more different diameter portions. The input piston 34includes a central bore 62 formed through the second end 52. One or morelateral passageways 64 are formed through the input piston 34. Thelateral passageways 64 extend from the outer cylindrical surface to thecentral bore 62. The brake pedal unit 20 is in a “rest” position asshown in FIGS. 1 and 2. In the “rest” position, the pedal 42 has notbeen depressed by the driver of the vehicle. In the rest position, thepassageways 64 of the input piston 34 are between the lip seals 58 and60. In this position, the passageways 64 are in fluid communication witha conduit 66 formed though the first portion 24 a of the housing 24. Theconduit 66 is in fluid communication with a conduit 68 formed in thesecond portion 24 b of the housing 24 which is in fluid communicationwith a reservoir port 70 connected to the reservoir 18. The conduits 66and 68 can be formed by various bores, grooves and passageways formed inthe first and second portions 24 a and 24 b of the housing 24. In therest position, the passageways 64 are also in fluid communication with aconduit 72 formed in the housing 24 which leads to a simulation valve 74(cut off valve which may be electrically operated). The simulation valve74 may be mounted in the housing 24, as shown in FIGS. 2-4, or may beremotely located therefrom, as schematically shown in FIG. 1.

The intermediate piston 36 is slidably disposed in the second bore 28 ofthe housing 24. The outer wall of the intermediate piston 36 is engagedwith a lip seal 80 mounted in a groove formed in the housing 24. Theintermediate piston 36 includes a first end 82 having a cavity 84 formedtherein. A second end 86 of the intermediate piston 36 includes a cavity88 formed therein. The cavity 88 includes a floor 90 defining a pivotgroove 92 which functions as a “rocker-like” or “cam-like” supportsurface for a wobble plate 94, which will be explained in further detailbelow.

The central bore 62 of the input piston 34 and the cavity 84 of theintermediate piston 36 house various components defining a pedalsimulator, indicated generally at 100. A caged spring assembly,indicated generally at 102, is defined by a pin 104, a retainer 106, anda low rate simulator spring 108. A first end 110 of the pin 104 is pressfit, threadably engaged, or otherwise attached to the input piston 34 ata bottom floor portion 111 of the central bore 62. The pin 104 extendsaxially through the central bore 62 and into the cavity 84 of theintermediate piston 36. A second end 112 of the pin 104 includes acircular flange 114 extending radially outwardly therefrom. A stop 123is provided which may be integrally formed with the retainer 120 or maybe a separate piece attached thereto. The stop 123 is axially alignedwith the pin 104. The stop includes a stem 125. A plurality of discsprings 118 (also known as Belleville washers, coned-disc springs, orcupped spring washers) is disposed on the stem 125 and retained thereonby a circlip 127. The disc springs 118 may be in a preloaded state whenmounted on the stem 125 or may mounted in a non-loaded state. Theretainer 106 of the caged spring assembly 102 includes a stepped throughbore 122. The stepped through bore 122 defines a shoulder 124. Thesecond end 112 of the pin 104 extends through the through bore 122. Theflange 114 of the pin 104 engages with the shoulder 124 of the retainer106 to prevent the pin 104 and the retainer 106 from separating fromeach other. One end of the low rate simulator spring 108 engages withthe second end 52 of the input piston 34, and the other end of the lowrate simulator spring 108 engages with the retainer 106 to bias theretainer 106 in a direction away from the pin 104.

The pedal simulator 100 further includes a high rate simulator spring130 which is disposed about the pin 104. The terms low rate and highrate are used for description purposes and are not to be limiting. Itshould be understood that that the various springs of the pedalsimulator 100 may have any suitable spring coefficient or spring rate.In the illustrated embodiment, the high rate simulator spring 130preferably has a higher spring rate than the low rate simulator spring108. A sleeve 132 is slidably disposed in the central bore 62 of theinput piston 34 and includes a through bore 134 through which the pin104 extends. The through bore 134 defines a radially inwardly extendingflange 136. One end of the high rate simulator spring 130 engages withthe flange 136. The other end of the high rate simulator spring 130 isshown in FIG. 2 in a non-engaged position and spaced away from an end ofthe retainer 106. A blend spring 140 is disposed about the pin 104. Oneend of the blend spring 140 engages with the flange 136 of the sleeve132, and the other end of the blend spring 140 engages with the bottomfloor portion 111 of the central bore 62 of the input piston 34. Thehousing 24, the input piston 34 (and its seals), and the intermediatepiston (and its seals) generally define a simulation chamber 144. Thesimulation chamber 144 is in fluid communication with a conduit 146which is in fluid communication with the simulation valve 74.

As discussed above, the brake pedal unit 20 includes the first andsecond output pistons 38 and 40 that are disposed in third and fourthbores 30 and 32, respectively, which are formed in the second portion 24b of the housing 24. The first and second output pistons 38 and 40 areslidably disposed along parallel axes. These axes are offset butparallel to the axis defined by the input piston 34 and the intermediatepiston 36. The third bore 30 includes a reduced diameter portion 150formed at a bottom floor 152 of the third bore 30. The reduced diameterportion 150 is in fluid communication with the reservoir 18 via areservoir conduit 154 and port 155. An output conduit 156 is formed inthe second portion 24 b of the housing 24 and is in fluid communicationwith the third bore 30 adjacent the bottom floor 152. The output conduit156 may be extended via external piping or a hose connected to thehousing 24. This output conduit 156 is shown schematically in FIG. 1.The fourth bore 32 includes a reduced diameter portion 160 formed at abottom floor 162 of the fourth bore 32. The reduced diameter portion 160is in fluid communication with the reservoir 18 via a reservoir conduit164 and port 165. An output conduit 166 is formed in the second portion24 b of the housing 24 and is in fluid communication with the fourthbore 32 adjacent the bottom floor 162. The output conduit 166 may beextended via external piping or a hose connected to the housing 24. Thisoutput conduit 166 is shown schematically in FIG. 1.

The first output piston 38 includes a lip seal 170 disposed in a grooveformed on the outer wall of the first output piston 38 and is sealinglyengaged with the wall of the third bore 30. The first output piston 38includes a through bore 172. The end 174 of the first output piston 38also includes an extension 178 attached thereto. The extension 178 maybe threadably attached to first output piston 38 and adjusted forlength. Set screws may be positioned in lateral passageways 176 to lockthe extension 178 in place. The extension 178 may include a seal 179 forsealing the fluid within the through bore 172. Alternatively, theextension 178 could be formed integrally with the first output piston38. The extension 178 includes a head portion 180 which engages with anend of the wobble plate 94, the reason for which will be explainedbelow. A pin 182 is slidably disposed in the through bore 172. One end184 of the pin 182 is disposed in the reduced diameter portion 150 ofthe third bore 30. Note that end 184 of the pin 182 and/or the reduceddiameter portion 150 may be formed with grooves or passageways toprovide selective fluid flow between the third bore 30 and the reservoirconduit 154. A compensation seal 186 is disposed at the end 184 of thepin 182 and cooperates with selective sealing against the bottom floor152 to provide a center valve assembly, indicated generally at 190 (seeFIG. 1). The other end 192 of the pin 182 includes an enlarged headportion 194 which prevents the pin 182 from separating from the firstoutput piston 38. An output piston spring 196 is disposed in the thirdbore 30 and biases the first output piston in a leftward direction, asviewing FIGS. 1 and 2. A first output pressure chamber 198 is defined bythe third bore 30, the first output piston 38, the seal 170 and thecenter valve assembly 190. The first output pressure chamber 198 is influid communication with the conduit 156. The first output pressurechamber 198 is also in selective fluid communication with the reservoirconduit 154 via the center valve assembly 190. A secondary springarrangement 199 may also be used to provide a bias of the compensationseal 186 against the bottom wall 152 and is used to initially get theelastomeric seal 186 to its seating position. Afterwards, pressurewithin the first output pressure chamber 198 helps keep the seal 186seated.

The arrangement of the second output piston 40 is similar to the firstoutput piston 38. The second output piston 40 includes a lip seal 200disposed in a groove formed on the outer wall of the second outputpiston 40 and is sealingly engaged with the wall of the fourth bore 32.The second output piston 40 includes a through bore 202. The end 204 ofthe second, output piston 40 also includes an extension 208 attachedthereto. The extension 208 may be threadably attached to the secondoutput piston 40 and adjusted for length. Set screws may be positionedin lateral passageways 206 to lock the extension 208 in place. Theextension 208 may include a seal 209 for sealing the fluid within thethrough bore 202. Alternatively, the extension 208 could be formedintegrally with the second output piston 40. The extension 208 includesa head portion 210 which engages with a second end of the wobble plate94, the reason for which will be explained below. A pin 212 is slidablydisposed in the through bore 202. One end 214 of the pin 212 is disposedin the reduced diameter portion 160 of the fourth bore 32. Note that end204 of the pin 212 and/or the reduced diameter portion 160 may be formedwith grooves or passageways to provide selective fluid flow between thefourth bore 32 and the reservoir conduit 164. A compensation seal 216 isdisposed at the end 214 of the pin 212 and cooperates with selectivesealing against the bottom floor 162 to provide a center valve assembly,indicated generally at 220. The other end 222 of the pin 212 includes anenlarged head portion 224 which prevents the pin 212 from separatingfrom the second output piston 40. An output piston spring 226 isdisposed in the fourth bore 32 and biases the second output piston 40 ina leftward direction, as viewing FIGS. 1 and 2. A second output pressurechamber 228 is defined by the fourth bore 32, the second output piston40, the seal 200, and the center valve assembly 220. The second outputpressure chamber 228 is in fluid communication with the conduit 166. Thesecond output pressure chamber 228 is also in selective fluidcommunication with the reservoir conduit 164 via the center valveassembly 220. A secondary spring arrangement 229 may also be used toprovide a bias of the compensation seal 220 against the bottom wall 162and is used to initially get the elastomeric seal 220 to its seatingposition. Afterwards, pressure within the second output pressure chamber228 helps keep the seal 220 seated.

Referring back to FIG. 1, the system 10 may further include a travelsensor, schematically shown at 240 in FIG. 1, for producing a signalthat is indicative of the length of travel of the input piston 34. Thesystem 10 may also include a switch 252 for producing a signal foractuation of a brake light and to provide a signal indicative ofmovement of the input piston 34. The brake system 10 may further includesensors such as pressure transducers 257 and 259 for monitoring thepressure in the conduits 164 and 166.

As discussed above, the power pack assembly 14 may be located remotelyfrom the hydraulic control unit 12. As will be discussed below, thepower pack assembly 14 generally provides a controlled source of fluidpressure (or boosted pressure) to the hydraulic control unit 12 via aboost conduit 260. The power pack assembly 14 includes a pump assembly262 driven by one or more motors 264. The pump assembly 262 can be anypump assembly capable of delivering the desired pressure levels. Forexample, the pump assembly 262 can be a single piston pump run by asingle motor. In the embodiment of the pump assembly 262 schematicallyshown in FIG. 1, the pump assembly 262 includes six pistons. The pumpassembly 262 may be configured into a pair of three pistonsub-assemblies 262 a and 262 b, having either one or two motors 264connected thereto. The three pistons may be offset by about 120 degreesfrom one another. The pump assembly 262 includes pump output conduits270 and 272 which are connected to a boost valve assembly, indicatedgenerally at 280. The boost valve assembly 280 can be configured as anyboost valve mechanism capable of providing fluid to the boost conduit260 at a desired pressure flow and pressure level. The boost valveassembly 280 may include a single boost valve or may include multipleindependently controlled valves. In the embodiment illustrated in FIG.1, the boost valve assembly 280 includes first, second, third, andfourth boost valves 282, 284, 286, and 288, respectively. The boostvalves 282, 284, 286, and 288 may be proportionally controlled solenoidvalves. The boost valves 282, 284, 286, and 288 also are fluidlyconnected to a conduit 290. The conduit 290 is in fluid communicationwith an optional reservoir 292 (or referred to as an accumulator). Theconduit 290 and the reservoir 292 are in fluid communication with thereservoir 18 via a reservoir conduit 296. The reservoir 292 stores fluidat a relatively low pressure and may be included in the system 10 toassist in providing a source of fluid for the inlet of the pump assembly262. Under certain circumstances, such as during low temperatures, itmay be desirable to provide a source of fluid at a relatively shortdistance from the inlet of the pump assembly 262 compared to drawingfluid from the reservoir 18 which may located a relatively far distanceaway. The power pack assembly 14 may also include a filter 299 in theconduit 296.

As stated above, the hydraulic control unit 12 includes a simulationvalve 74 which may be mounted in the housing 24, as shown in FIGS. 2-4.As schematically shown in FIG. 1, the simulation valve 74 may be asolenoid actuated valve. The simulation valve 74 includes a first port300 and a second port 302. The port 300 is in fluid communication withthe conduit 146 which is in fluid communication with the simulationchamber 144. The port 302 is in fluid communication with the conduit 72which is in fluid communication with the reservoir 18 via the conduits66 and 68. The simulation valve 74 is movable between a first position74 a restricting the flow of fluid from the simulation chamber 144 tothe reservoir 18, and a second position 74 b permitting the flow offluid between the reservoir 18 and the simulation chamber 144.

The system 10 further includes a first base brake valve 320 and a secondbase brake valve 322 (also referred to as switching valves or switchingvalve arrangement). The base brake valves 320 and 322 may be solenoidactuated three way valves. The base brake valves 320 and 322 aregenerally operable to two positions, as schematically shown in FIG. 1.The first base brake valve 320 has a port 320 a in fluid communicationwith the conduit 156 which is in fluid communication with the firstoutput pressure chamber 198. A port 320 b is in fluid communication withthe boost conduit 260. A port 320 c is in fluid communication with aconduit 324 which is selectively in fluid communication with the wheelbrakes 16 b and 16 c. The second base brake valve 322 has a port 322 ain fluid communication with the conduit 166 which is in fluidcommunication with the second output pressure chamber 228. A port 322 bis in fluid communication with the boost conduit 260. A port 322 c is influid communication with a conduit 326 which is selectively in fluidcommunication with the wheel brakes 16 a and 16 d.

The system 10 further includes various valves (slip control valvearrangement) for permitting controlled braking operations, such as ABS,traction control, vehicle stability control, and regenerative brakingblending. A first set of valves include an apply valve 340 and a dumpvalve 342 in fluid communication with the conduit 324 for cooperativelysupplying brake fluid received from the boost valve assembly 280 to thewheel brake 16 c, and for cooperatively relieving pressurized brakefluid from the wheel brake 16 c to a reservoir conduit 343 in fluidcommunication with the reservoir conduit 72. A second set of valvesinclude an apply valve 344 and a dump valve 346 in fluid communicationwith the conduit 324 for cooperatively supplying brake fluid receivedfrom the boost valve assembly 280 to the wheel brake 16 b, and forcooperatively relieving pressurized brake fluid from the wheel brake 16b to the reservoir conduit 343. A third set of valves include an applyvalve 348 and a dump valve 350 in fluid communication with the conduit326 for cooperatively supplying brake fluid received from the boostvalve assembly 280 to the wheel brake 16 d, and for cooperativelyrelieving pressurized brake fluid from the wheel brake 16 d to thereservoir conduit 343. A fourth set of valves include an apply valve 352and a dump valve 354 in fluid communication with the conduit 326 forcooperatively supplying brake fluid received from the boost valveassembly 280 to the wheel brake 16 a, and for cooperatively relievingpressurized brake fluid from the wheel brake 16 a to the reservoirconduit 343.

The following is a description of the operation of the brake system 10.FIGS. 1 and 2 illustrate the brake system 10 and the brake pedal unit 20in the rest position. In this condition, the driver is not depressingthe brake pedal 42. Also in the condition, the simulation valve 74 maybe energized or not energized. During a typical braking condition, thebrake pedal 42 is depressed by the driver of the vehicle. The brakepedal 42 is coupled to the travel sensor 240 for producing a signal thatis indicative of the length of travel of the input piston 34 andproviding the signal to an electronic control module (not shown). Thecontrol module can include a microprocessor. The control module receivesvarious signals, processes signals, and controls the operation ofvarious electrical components of the brake system 10 in response to thereceived signals. The control module can be connected to various sensorssuch as pressure sensors, travel sensors, switches, wheel speed sensors,and steering angle sensors. The control module may also be connected toan external module (not shown) for receiving information related to yawrate, lateral acceleration, longitudinal acceleration of the vehiclesuch as for controlling the brake system 10 during vehicle stabilityoperation. Additionally, the control module may be connected to theinstrument cluster for collecting and supplying information related towarning indicators such as ABS warning light, brake fluid level warninglight, and traction control/vehicle stability control indicator light.

During normal braking operations the pump assembly 262 and the boostvalve assembly 280 are usually operated to provide boost pressure to theboost conduit 260 for actuation of the wheel brakes 16 a-d. The boostconduit 260 provides pressurized fluid to the conduits 324 and 326 viathe energized base brake valves 320 and 322. Under certain drivingconditions, the control module communicates with a powertrain controlmodule (not shown) and other additional braking controllers of thevehicle to provide coordinated braking during advanced braking controlschemes (e.g., anti-lock braking (AB), traction control (TC), vehiclestability control (VSC), and regenerative brake blending). The controlmodule actuates the boost valves 282, 284, 286, and 288 to provide adesired boost pressure level and fluid flow to the boost conduit 260.The control module may control the boost valves 282, 284, 286, and 288in various different ways to provide the desired pressure level at theboost conduit 260. The boost valves 282, 284, 286, and 288 areselectively actuated to divert fluid from the pump outlet conduits 270and 272, and selectively actuated to divert fluid to the reservoirconduit 290 to obtain the desired fluid flow and pressure level in theboost conduit 260. Ideally, the boost valves 282, 284, 286, and 288 areactuated to provide a relatively quiet operation of the components ofthe power pack assembly 14 and in an energy efficient manner. Althoughfour boost valves are shown, it should be understood that the boostvalves may be replaced with just a pair of proportionally controlledvalves. Thus, use of two pairs of boost valves 282, 284, 286, and 288may be desirable such that smaller less inexpensive valves can be used.A pressure transducer 360 may be connected to the boost conduit 260 toprovide a signal for the control module indicative of the pressure levelwithin the conduit 260. The pressure transducer 360 may be located ineither of the hydraulic control unit 12 or the power pack assembly 14.

During a normal boost apply braking operation, such as is shown in FIG.3, the flow of pressurized fluid from the brake pedal unit 20 generatedby depression of the brake pedal 42 is diverted into the pedal simulatorassembly 100. The simulation valve 74 is actuated to divert fluidthrough the simulation valve 74 from the simulation chamber 144 to thereservoir 18 via the conduits 146, 72, 66, and 68. Note that fluid flowfrom the simulation chamber 144 to the reservoir 18 is closed off oncethe passageways 64 move past the seal 60. Prior to movement of the inputpiston 34, as shown in FIG. 2, the simulation chamber 144 is in fluidcommunication with the reservoir 18 via the conduits 66 and 68.

The base brake valves 320 and 322 are energized to a secondary positionto prevent the flow of fluid from the conduits 156 and 166 through thevalves 320 and 322. Fluid flow is prevented from flowing from the ports320 a and 322 a to the ports 320 c and 322 c, respectively. Thus, thefluid within the first and second output pressure chambers 198 and 228are fluidly locked which generally prevents the first and second outputpistons 38 and 40 from moving further. More specifically, during theinitial stage of the normal boost apply braking operation, movement ofthe input rod 45 causes movement of the input piston 34 in a rightwarddirection, as viewing FIG. 2. Initial movement of the input piston 34causes movement of the intermediate piston 36 via the low rate simulatorspring 108. Movement of the intermediate piston 36 causes initialmovement of the first and second output pistons 38 and 40 via the wobbleplate 94. The wobble plate 94 is free to pivot in a relatively smallamount to help equalize pressures within the first and second outputchambers 198 and 228. The first and second output pistons 38 and 40 moveuntil their center valve assemblies 190 and 220 close. Morespecifically, the center valve assemblies 190 and 220 close when theseals 186 and 216 seal against the floors 152 and 162, respectively.

After the center valve assemblies 190 and 220 have closed, the inputpiston 34 continues to move rightward, as viewing FIG. 2, upon furthermovement by the driver depressing the brake pedal 42. Further movementof the brake pedal 42 by the driver will cause the input piston 34 tomove rightward, as viewing FIG. 2, compressing the various springs ofthe pedal simulator assembly 100, thereby providing a feedback force tothe driver of the vehicle. The description of the operation of the pedalsimulator 100 will be described with respect to the graph of FIG. 5,wherein the line represents the pedal force vs. pedal travel for anembodiment of invention, such as illustrated in FIGS. 1-4. The pedalforce corresponds to the feedback force the driver feels at their foot.It should be understood that the numerical data is not meant to belimiting to the invention and describes only one possible embodiment ofthe invention. The force of about 20 Newtons prior to travel correspondsto a knee point 370 which corresponds to the preload of the springs andvarious seals such as those associated with the output pistons 38 and40. The region A generally corresponds to the travel until the centervalve assemblies 190 and 220 close off, at about 4 or 5 mm of travel, asindicated by the knee point 371. At this point, the output springs 196and 226 generally stop compressing. If desired, the various springs ofthe pedal simulator 100 and the output springs could be configured suchthat the knee point 371 point between region A and B is not detectableby the driver. During the region B of FIG. 5, the input piston 34 willcontinue moving compressing the low rate simulator spring 108 until theright-hand end of the high rate simulator spring 130 contacts theretainer 106 corresponding to the knee point 372. Further movementcauses the high rate simulator spring 130 and the blend spring 140 tocompress corresponding to the region C. The blend spring 140 willcontinue to compress until the sleeve 132 contacts the bottom floorportion 111 of the central bore 62 of the input piston 34 whichcorresponds to the knee point 373. Further movement, now correspondingto region D, will compress the high rate simulator spring 130 and thelow rate simulator spring 108 until the disc springs 118 contact the end112 of the pin 104 at a knee point 374. Further movement, nowcorresponding to region E, will also compress the disc springs 118 aswell as the high rate simulator spring 130 and the low rate simulatorspring 108. Further movement will cause the stem 125 of the stop 123 tocome into direct contact with the end 112 of the pin 104. At this point,the input piston 34 is in a “hard” engagement with the intermediatepiston 36 via the pin 104 such that there is no further compression ofthe springs 108, 130, and 118. The springs may be configured such thatthey may or may not be fully compressed at this time. In an alternateembodiment, the disc springs 118 could be replaced with an elastomericspring element, such as by replacing the rigid stop 123 with anelastomeric spring element. The end of the pin 104 (which may bereconfigured) would then compress the elastomeric spring element.

During operation of the power pack 14, the pistons of the pump assembly262 may all be supplying fluid to the conduits 270 and 272. Forcircumstances when a lower fluid pressure rate is desired, the pumpassembly 262 could be operated in an energy saving mode to reduce theelectrical consumption of the motor 264. In this mode, the pumpsub-assembly 262 a provides an output pressure to the conduit 260, butthe other pump sub-assembly 262 b does not. A check valve 367 may beprovided to prevent fluid flow from the outlet of the pump sub-assembly262 a via the conduit 270 from entering the conduit 272. In this energysaving mode, the third and fourth boost valves 286 and 288 could beopened to allow fluid to freely loop from the outlet to the inlet of thepump sub-assembly 262 b, thereby putting a relatively low load on thepump assembly 262 b compared to the load of the pump assembly 262 a. Itshould be understood that the pump assemblies 262 a and 262 b may havefewer than three pistons or more than three pistons each. Also, the pumpassemblies 262 a and 262 b may be operated by a single motor or by twoor more motors.

In the event of a loss of electrical power to portions of the brakesystem 10, the brake system 10 provides for manual push through ormanual apply, such as is shown in FIG. 4. During an electrical failure,the motor 264 might cease to operate, thereby failing to producepressurized hydraulic brake fluid from the pump assembly 262.Furthermore, the boost valve assembly 280 might return to a de-energizedposition if energized. In this situation, the power pack assembly 14does not supply the desired pressurized fluid within the boost conduit260. The base brake valves 320 and 322 will shuttle to the positionsshown in FIG. 1 closing off fluid flow from the boost conduit 260 to theconduits 324 and 326. In these positions, the base brake valves 320 and322 permit fluid flow from the conduits 156 and 166 (via the ports 320 aand 322 a) to the conduits 324 and 326 (via the ports 320 c and 322 c),respectively. Thus, the brake pedal unit 20 may now provide manual applyfor energizing the fluid conduits 324 and 326 for actuation of the wheelbrakes 16 a-d. The simulation valve 74 is shuttled to its position asshown in FIG. 1 to prevent fluid from flowing out of the simulationchamber 144 to the reservoir 18, thereby hydraulically locking thesimulation chamber 144. During manual apply as shown in FIG. 4, thefirst and second output pistons 38 and 40 will advance rightwardpressurizing the chambers 198 and 228. Fluid flows from the chambers 198and 228 into the conduits 324 and 326, respectively, to actuate thewheel brakes 16 a-d. Note that the fluid within the simulation chamber144 is trapped or locked and movement of the input piston 34 will causemovement of the intermediate piston 36 which causes movement of thefirst and second output pistons 38 and 40 via the wobble plate 94. Dueto differences in hydraulic effective areas of the input andintermediate pistons 34 and 36, the input piston 34 may travel moreaxially than the intermediate piston 36. Although a reduced diametereffective area of the input piston 34 compared to the larger diametereffective area of the intermediate piston 36 requires further travel,the force input by the driver's foot is reduced.

In another example of a failed condition of the brake system 10, thepower pack assembly 14 may fail as discussed above and furthermore oneof the output pressure chambers 198 and 228 may be reduced to zero orreservoir pressure, such as failure of a seal. The wobble plate 94 willaccordingly pivot on one side until it bottoms out against the floor 90.However, since the wobble plate 94 and its corresponding pivot groove 92are configured such that the wobble plate 94 cannot pivot by a largeamount, the driver will not feel much of a pedal drop when one of thecircuits fails. In this manual push through situation, the pressurewithin the other non-failed output pressure chamber 198 or 228 will beat about twice the pressure.

As stated above with respect to FIG. 1, the simulation valve 74 may be asolenoid actuated valve movable between a first position 74 arestricting the flow of fluid from the simulation chamber 144 to thereservoir 18, and a second position 74 b permitting the flow of fluidbetween the reservoir 18 and the simulation chamber 144. In theembodiment illustrated in FIGS. 2-4, the simulation valve 74 operates incooperation with an optional check valve assembly 380 located in theconduit 72. The assembly 380 includes a ball 384 and seat 385 which isin a parallel arrangement with a restricted orifice 386. The assembly380 may be retained in the housing 24 by a retainer 388. During a spikeapply in which the driver presses on the brake pedal 42 in a rapid andforceful manner, the damping orifice 386 restricts the flow of fluidthrough the conduit 72 from the simulation chamber 144, thereby impedingadvancement of the input piston 34. This may be a desirablecharacteristic of the brake system 10 as compared to a system in whichthe fluid can rapidly flow out of the simulation chamber 144. The sizeof the damping orifice can be sized accordingly. Note that the one waycheck valve assembly 380 provides for bypass flow path in the directionfrom the reservoir 18 to the simulation valve 74, thereby bypassing thedamping orifice 386. The function of the damping orifice 386 and thecheck valves assembly 380 may be integrally provided in the simulationvalve 74 instead of being separate components.

The simulation valve 74 may be configured as a simple digital open/closevalve arrangement, as indicated schematically at 74 in FIG. 1.Alternatively, the simulation valve 74 may be configured as a dual stagevalve (two stage valve), as is shown in the embodiment shown in FIGS.2-4. In this embodiment, the simulation valve 74 includes a firstsealing assembly 390 defined by a slidably disposed poppet 392 whichengages with a seat 393. The poppet 392 includes a relatively large bore394 formed therethrough and includes a restricted orifice 395 on oneend. A second sealing assembly 396 is situated at the restricted orifice395 and includes a ball seat arrangement mounted on an armature 397.Note that the orifice 386 may be larger than the orifice 395 within thesimulation valve 74.

During normal braking, the simulation valve 74 is actuated such that thearmature 397 moves rightward, as shown in FIG. 3, opening the firstsealing assembly 90 and the second sealing assembly 396 to permit theflow of fluid from the conduit 146 through the simulation valve 74 andinto the conduit 72 in a generally unrestricted manner. The inclusion ofthe dual stage simulation valve 74 provides for a reduced pedal dropexperienced by the driver under certain situations. For example, if thedriver were to depress the brake pedal 42 when the vehicle was notturned on, i.e., no electrical power being supplied to the system 10,the system 10 enters into a manual apply mode, as shown in FIG. 4. Inthis situation, the simulation valve 74 is de-energized, thereby closingthe simulation valve and trapping the fluid within the simulationchamber 144. The driver is able to advance the input piston 34, theintermediate piston 36, and the output pistons 38 and 40 as describedabove. Note that the pressure within the simulation chamber 144increases the more the driver advances the input piston 34 via the brakepedal 42 due in part because of reactionary forces from the wheel brakes16 a-d. If the driver then turns on the ignition while still applyingpressure to the brake pedal 42, the simulation valve 74 can be energizedwith enough current to open the second sealing assembly 396 but not thefirst sealing assembly 390. Thus, the poppet 392 remains on the seat 393due to a pressure build up within the simulation valve 74. Pressurizedfluid from the simulation chamber 144 is then diverted through therestricted orifice 395 of the simulation valve 74. This restrictionimpedes the flow of fluid such that the input piston 34 will notsuddenly or rapidly advance towards the intermediate piston 36 due thecontinuing force applied by the driver.

There is illustrated in FIG. 6 a second embodiment of a vehicle brakesystem indicated generally at 400. Similar to the above described brakesystem 10, the brake system 400 may suitably be used on a ground vehiclesuch as an automotive vehicle having four wheels and a wheel brake foreach wheel. Furthermore, the brake system 400 can be provided with otherbraking functions such as anti-lock braking (ABS), other slip controlfeatures, and regenerative braking blending to effectively brake thevehicle. The brake system 400 is similar in function and structure ofsome aspects of the brake system 10 and, therefore, like numbers and ornames may be used to reference similar components.

The brake system 400 generally includes a hydraulic control unit whichmay be the same hydraulic control unit 12 as described above withrespect to FIGS. 1-5. One of the differences between the systems 10 and400 is that the brake system 400 uses a different power pack assembly414. The power pack assembly 414 provides a source of pressurized fluidfor the system 400 via a boost conduit 260.

The power pack assembly 414 includes a pump assembly 420 driven by amotor 422. The pump assembly 420 may have any configuration, such as athree piston configuration schematically illustrated in FIG. 6. Thepower pack assembly 414 further includes a medium pressure accumulator(MPA) 424 and a medium pressure accumulator (MPA) priority valve 426.The reservoir conduit 296 provides hydraulic braking fluid from thereservoir 18 to the pump assembly 420. In one embodiment, the motor 422is a flux switching brushless motor that self monitors its torqueoutput. The MPA 424 is in selective fluid communication with the outletof the pump assembly 420 and the MPA priority valve 426 via fluidconduits 430 and 432. The conduit 430 may include a filter 434.

The MPA 424 may be a piston style accumulator including an internalpiston biased by a spring. Such a medium pressure accumulator isdisclosed and described in PCT Patent Publication No. WO 2009/058916,which is incorporated by reference herein. A chamber is defined by thepiston and wall portions of a housing of the MPA 424. The spring biasesthe piston in a direction to reduce the volume of the chamber, therebypressurizing the chamber and the fluid conduit 430. The MPA 424 ispreferably an accumulator that is capable of storing pressurized fluidto a predetermined pressure (operating pressure). Although the MPA 424is referred to as a “medium” pressure accumulator as opposed to a “high”pressure accumulator used in conventional brake systems, the MPA 424 maybe configured to store pressurized fluid at any desired pressure levels.The MPA 424 preferably includes a bypass function that dumps fluid tothe reservoir 18 via a conduit 436 and the reservoir conduit 296 whenthe pressure within the MPA 424 exceeds a predetermined threshold value.The pressurized fluid in the MPA 424 is utilized to supply pressurizedfluid to a boost valve 440 for braking demands that the vehicleencounters during a normal course of driving. This is opposed toemergency braking in which demand for fluid pressure at the boost valve440 requires a relatively high amount of pressurized brake fluid(pressure exceeding the bypass pressure of the MPA 424) wherein the pumpassembly 420 provides for the demanded higher pressure levels.

In operation, the brake pedal unit 12 of the system 400 operates in asimilar manner as the system 10. During operation in a boost applystate, pressurized brake fluid from the pump assembly 420 and/or the MPA424 of the system 400 is supplied to the electro-hydraulic boost valve440. The boost valve 440 may be a proportional pressure control spoolvalve or a proportional flow control spool valve that at times maymaintain pressure in the fluid conduit 432 to allow pressurized brakefluid to flow from the discharge of the pump assembly 420 and/or to theMPA 424 for pressurizing the MPA 424 with pressurized brake fluid. Theboost valve 440 further allows the flow of pressurized brake fluid viathe boost conduit 260 for actuating the wheel brakes 16 a-d in a similarmanner as described above with respect to the system 10.

The MPA priority valve 426 is disposed between the pump assembly 420 andthe MPA 424. The MPA priority valve 426 controls the supply ofpressurized brake fluid from the pump assembly 420 to the MPA 424 forcharging the MPA 424. The MPA priority valve 426 is a pilot operatedvalve referencing the boost pressure in the boost conduit 260 and thepressure exiting the pump assembly 420. For high demand brakingoperations in which the pressure required from the boost valve 440exceeds, for example a pressure slightly higher than the predeterminedpressure, which is above the bypass pressure of the MPA 424, the MPApriority valve 426 will actuate between open, closed, and meteringpositions to provide sufficient pressure from the pump assembly 420 inexcess of the boost pressure demanded in the conduit 260.

A normally closed (N/C) MPA valve 450 is disposed in the conduit 432.The N/C MPA valve 450 may be a two-position digital isolation valveoperable between a fully open position and a fully closed position. TheN/C MPA valve 450 may be current limited depending on the pressure andflow rates acting on the valve. This provides the advantage ofconserving energy for maintaining the N/C closed valve 450 in an openposition when maximum current is not required to do so.

When in a closed position, the N/C MPA valve 450 prevents leakage acrossthe boost valve 440 when the boost valve 670 is in an unactuated state.This allows the boost valve 440 to be manufactured with lower toleranceswhich reduces the cost of the boost valve 440. Conventionally, spoolvalves used in boost valves are typically machined to high tolerances tominimize leakage through the boost valve when closed. With the inclusionof the N/C MPA valve 450, the system 400 may include a lower cost boostvalve 440. For example, when the boost valve 440 is in an unactuatedposition, the boost valve 440 isolates the pressurized fluid generatedby the pressure source from the boost conduit 260. Leakage of fluidthrough the spool valve of the boost valve 440 when the boost valve 440is in an unactuated state results in a loss of pressure in the MPA 424.The pump assembly 420 would thereafter intermittently operate tomaintain the pressure in the MPA 424 which ultimately consumes energy asused by the motor. When the N/C MPA valve 450 is in a closed position,the boost valve 440 is isolated from the pressure exerted by the MPA 424which could otherwise result in leakage. As a result, the boost valve440 may be manufactured at lower tolerances which ultimately reduces thecost of the boost valve 440 and greatly minimizes energy consumptionsince the motor 422 will not have to operate as often to fill thedepleting MPA 424.

The power pack 414 of the system 400 may also include various componentsfor assisting in evacuation and filing the brake system 400 with fluid,such as for example a restricted orifice 453 and a valve 454. The systemmay further include a valve 452 for manually depleting the MPA 424 ifthere is no power to the system 400.

There is illustrated in FIG. 7 a third embodiment of a vehicle brakesystem indicated generally at 460. Similar to the above described brakesystems 10 and 400, the brake system 460 may suitably be used on aground vehicle such as an automotive vehicle having four wheels and awheel brake for each wheel. Furthermore, the brake system 460 can beprovided with other braking functions such as anti-lock braking (ABS),other slip control features and regenerative brake blending toeffectively brake the vehicle. The brake system 460 is similar infunction and structure to the brake system 400 (and the brake system 10)and, therefore, like numbers and or names may be used to referencesimilar components.

The brake system 460 generally includes the same components as the brakesystem 400 illustrated in FIG. 6, but packages the componentsdifferently. In the brake system 460, a hydraulic control unit 462includes the brake pressure unit 20, the reservoir 18, and thesimulation valve 74. The remainder of the components may be housed in acommon block 464. This arrangement may provide packaging advantages overthe brake system 400 in which the space available at the brake pedalunit 20 mounting location is limited. Another advantage is that thebrake system 460 may need only one electronic control unit.

There is illustrated in FIG. 8 an alternate embodiment of a vehiclebrake system indicated generally at 500. Similar to the above describedbrake systems, the brake system 500 may suitably be used on a groundvehicle such as an automotive vehicle having four wheels and a wheelbrake for each wheel. Furthermore, the brake system 500 can be providedwith other braking functions to effectively brake the vehicle.

The brake system 500 includes a master cylinder assembly, indicatedgenerally at 502 in FIG. 8, which is also shown enlarged in FIG. 9. Thebrake system 500 further includes a brake module, indicated generally at504 in FIG. 8. The components of the brake module 504 may be housed inone or more hydraulic control blocks and may be located remotely fromthe master cylinder assembly 502. Conduits or hydraulic lines mayhydraulically couple the master cylinder assembly 502 and the brakemodule 504.

The master cylinder assembly 504 cooperatively acts with the brakemodule 504 for actuating wheel brakes 506 a, 506 b, 506 c, and 506 d.The wheel brakes 506 a, 506 b, 506 c, and 506 d can be associated withany combination of front and rear wheels of the vehicle in which thebrake system 500 is installed. For example, wheel brakes 506 a and 506 bmay be associated with the front wheels and wheel brakes 506 c and 506 dmay be associated with rear wheels. Alternatively, for a diagonallysplit brake system, the wheel brakes 506 a and 506 c may be associatedwith the front wheels and the wheel brakes 506 b and 506 d may beassociated with the rear wheels. Each of the wheel brakes 506 a, 506 b,506 c, and 506 d may be a conventional brake operated by the applicationof pressurized brake fluid. The wheel brake may be, for example, a brakecaliper mounted on the vehicle to engage a frictional element (such as abrake disc) that rotates with a vehicle wheel to effect braking of theassociated vehicle wheel.

As schematically shown in FIG. 9, the master cylinder assembly 502includes a master cylinder 510 in fluid communication with a reservoir512. The reservoir 512 generally holds hydraulic fluid at atmosphericpressure. The master cylinder 510 includes a housing 514 having variousbores formed therein for slidably receiving various cylindrical pistonstherein. The master cylinder housing 514 may be formed as a single unitor two or more separately formed portions coupled together. The mastercylinder 510 generally includes a first bore 518, a second bore 520, athird bore 522, and a fourth bore 524. The first and second bores 518and 520 are axially aligned with one another. The third and fourth bores522 and 524 are in communication with the larger diameter second bore520. The third bore 522 is located above the fourth bore 522 as viewingFIG. 12. As will be discussed below, a primary piston 526 is slidablydisposed in the first and second bores 518 and 520. A first outputpiston 530 is slidably disposed in the third bore 522. A second outputpiston 532 is slidably disposed in the fourth bore 524.

A brake pedal 536 is coupled to a first end of a primary piston 526 ofthe master cylinder 510 via an input rod 538 and a retainer 540. Theretainer 540 is disposed in a bore 542 formed in the primary piston 526and is biased by a spring 544. The system 500 may further include atravel sensor 713 for producing a signal that is indicative of thelength of travel of the input rod 538 and/or the length of travel of theprimary piston 526. For example, a magnet may be mounted on the primarypiston 526, the position of which is detectable by the travel sensor713. The travel sensor may alternatively be connected to the input rod538 as describe above. The system 500 may also include a switch 537 forproducing a signal for actuation of a brake light and to provide asignal indicative of movement of the input piston 34.

The primary piston 526 includes a first portion 546 slidably disposed inthe first bore 518, and a second portion 548 slidably disposed in thesecond bore 520. The outer wall of the first portion 546 is sealinglyengaged with seals 550 and 552 mounted in grooves formed in the housingat the first bore 518. A conduit 554 is in fluid communication with thefirst bore 518 between the seals 550 and 552. The other end of theconduit 554 connects to the reservoir 512.

As viewing FIGS. 8 and 9, the left-hand end of the first output piston530 includes an enlarged head 556 which is retained in a slot 558 formedin the second portion 548 of the primary piston 526. The primary piston526 includes a shoulder 560 formed adjacent the slot 558. The enlargedhead 556 is retained in the slot 558 and is prevented from moving in arightward direction relative to the primary piston 526 by the shoulder560 when the pistons 526 and 530 are positioned as shown in FIG. 9.Similarly, the left-hand end of the second output piston 532 includes anenlarged head 562 which is retained in a slot 564 formed in the secondportion 548 of the primary piston 526. The primary piston 526 includes ashoulder 566 formed adjacent the slot 564. The enlarged head 562 isretained in the slot 564 and is prevented from moving in a rightwarddirection relative to the primary piston 526 by the shoulder 560 whenthe pistons 526 and 532 are positioned as shown in FIG. 9. A spring 568biases the first output piston away from the primary piston 526. Aspring 570 biases the second output piston away from the primary piston526.

The first output piston 530 includes seals 572 and 574 for sealinglyengaging with the walls of the third bore 522. The second output piston532 includes seals 576 and 578 for sealingly engaging with the walls ofthe fourth bore 524. In general terms, a primary chamber 580 is definedby the second bore 520 between the seal 552 of the primary piston andthe seals 572 and 576 of the first and second output pistons 530 and532. However, the primary chamber 580 is also defined by the bores 518,522, 524, the primary piston 526, the first and output pistons 530 and532. The primary chamber 580 is in fluid communication with thereservoir 512 via a conduit 582. A normally closed service bleed screw584 may be used for selectively opening the conduit 582 such as duringservice bleeding of the brake system 500. The primary chamber 580 isalso in fluid communication with a pedal simulator assembly 610 via aconduit 588, as will be discussed below.

A first output chamber 590 is generally defined by the third bore 522,the seal 574 and the right-hand end of the first output piston 530, asviewing FIG. 9. The first output chamber 590 is in fluid communicationwith the reservoir 512 via a conduit 592. A poppet style compensationport valve assembly 594 mounted on the end of the first output piston530 selectively closes off fluid communication between the first outputchamber 590 and the conduit 592 when the first output piston 530 moves asufficient distance in the rightward direction as viewing FIG. 9. Areturn spring 596 biases the first output piston 530 leftward towardsthe primary piston 526. The first output chamber 590 is in fluidcommunication with a conduit 599 in fluid communication with the brakemodule 504.

Similarly, a second output chamber 600 is generally defined by thefourth bore 524, the seal 578 and the right-hand end of the secondoutput piston 532, as viewing FIG. 9. The second output chamber 600 isin fluid communication with the reservoir 512 via a conduit 602. Apoppet style compensation port valve assembly 604 mounted on the end ofthe second output piston 532 selectively closes off fluid communicationbetween the second output chamber 600 and the conduit 602 when thesecond output piston 532 moves a sufficient distance in the rightwarddirection as viewing FIG. 12. A return spring 606 biases the secondoutput piston 532 leftward towards the primary piston 526. The secondoutput chamber 600 is in fluid communication with a conduit 608 in fluidcommunication with the brake module 504. The reservoir 512 is in fluidcommunication with the brake module 504 via a reservoir conduit 609.

The pedal simulator assembly 610 may be mounted in the housing 514, asshown in FIG. 9, or may be located remotely. The pedal simulatorassembly 610 includes a piston 612 slidably disposed in a bore 614 ofthe housing 514. A seal 616 engaged between the piston 612 and the wallof the bore 614 generally separates the bore 614 into a pressure chamber618 and a reservoir chamber 620. The pedal simulator assembly 610further includes a collar 622 slidingly disposed on a stem 624 of thepiston 612. A collar spring 626 biases the collar 622 away from the mainportion of the piston 612. A return spring 628 biases the collar 622 andthe piston 612, via the collar spring 626, in the left-hand direction asviewing FIG. 9. The return spring 628 preferably has a higher springrate than the collar spring 626.

The pressure chamber 618 of the pedal simulator 610 is in fluidcommunication with the primary chamber 580 via the conduit 588. Thereservoir chamber 620 of the pedal simulator 610 is in selective fluidcommunication with the reservoir 512 via a conduit 630. As shown in FIG.9, the conduit 630 is in fluid communication with the third bore 522between the seals 572 and 574 of the first output piston 530. Theconduit 630 is also in fluid communication with the fourth bore 524between the seals 576 and 578 of the second output piston 532. A conduit632 is in fluid communication with the third bore 522 between the seals572 and 574 of the first output piston 530. The conduit 632 is also influid communication with the fourth bore 524 between the seals 576 and578 of the second output piston 532. The conduit 632 is in fluidcommunication with the reservoir 512 via the conduit 554. A reduced areaorifice 636 may be within the conduit 632 for limiting the flow of fluidthrough the conduit 632. A check valve 638 may be disposed in parallelwith the conduit around the orifice to permit a relative large flow offluid in a direction from the reservoir 512 to the conduit 632.

Referring to FIG. 8, the brake module 504 may be located remotely fromthe master cylinder assembly 502. Many of the components of the brakemodule 504 are similar in function as the components of the systemsdescribed above. The brake module 504 includes a pressure source,indicated generally at 650. The pressure source 650 provides a source ofpressurized hydraulic fluid to the system 500. The pressure source 650generally includes a pump 652 driven by a motor 654, a medium pressureaccumulator (MPA) 656, and a medium pressure accumulator (MPA) pilotvalve 658. The pump 652, the motor 654, the MPA 656, and the MPA pilotvalve 658 may all function in a similar manner and have the samestructure as those corresponding components discussed above. Thereservoir conduit 609 provides hydraulic braking fluid from thereservoir 512 to the pump 652. The pump 652 may be a three-piston stylepump driven by an eccentric and oriented 120 degrees from one another.In one embodiment, the motor 654 is a flux switching brushless motorthat self monitors its torque output. The MPA 656 is in selective fluidcommunication with the outlet of the pump 652 and the MPA pilot valve658 via fluid conduits 659 and 660. The conduit 660 may include a filter663.

The MPA 656 may have any suitable structure capable of storing fluid toa predetermined pressure pressure. The MPA 656 may be a piston styleaccumulator including an internal piston biased by a spring. A chamberis defined by the piston and wall portions of a housing of the MPA 656.The spring biases the piston in a direction to reduce the volume of thechamber, thereby pressurizing the chamber and the fluid conduit 659.Although the MPA 656 is referred to as a “medium” pressure accumulatoras opposed to a “high” pressure accumulator used in conventional brakesystems, the MPA 656 may be configured to store pressurized fluid at anydesired pressure levels. The MPA 656 preferably includes a bypassfunction that dumps fluid to the reservoir 512 via a conduit 662 and thereservoir conduit 609 when the pressure within the MPA 656 exceeds apredetermined threshold value. The pressurized fluid in the MPA 656 isutilized to supply pressurized fluid to a boost valve 670 for brakingdemands that the vehicle encounters during a normal course of driving.This is opposed to emergency braking in which demand for fluid pressureat the boost valve 670 requires a relatively high amount of pressurizedbrake fluid (pressure exceeding the bypass pressure of the MPA 656)wherein the fluid pump 652 provides for the demanded higher pressurelevels.

In operation, pressurized brake fluid from the pump 652 and/or the MPA656 is supplied to the electro-hydraulic boost valve 670. The boostvalve 670 may be a variable flow spool valve that at times may maintainpressure in the fluid conduit 660 to allow pressurized brake fluid toflow from the discharge of the pump 652 the MPA 656 for pressurizing theMPA 656 with pressurized brake fluid. The boost valve 670 further allowsthe flow of pressurized brake fluid via a boost conduit 672 foractuating the wheel brakes 506 a-d.

The MPA pilot valve 658 is disposed between the pump 652 and the MPA656. The MPA pilot valve 658 controls the supply of pressurized brakefluid from the pump 652 to the MPA 656. The MPA pilot valve 658 is apilot operated valve referencing the boost pressure in the boost conduit672 and the pressure exiting the pump 652. During normal brakingoperations in which the fluid pressure from the pump 652 and the MPA 656are below a predetermined pressure, the MPA pilot valve 658 will be inan open position permitting fluid communication between the pump 652 andthe MPA 656. For high demand braking operations in which the pressurerequired from the boost valve 670 exceeds, for example a pressureslightly higher than the predetermined pressure, which is above thebypass pressure of the MPA 656, the MPA pilot valve 658 will actuatebetween open, closed, and metering positions to provide sufficientpressure from the pump 652 in excess of the boost pressure demanded inthe circuit 672.

A normally closed (N/C) MPA valve 680 is disposed between a port 670 aof the boost valve and the conduit 660 from the pressure source 650. TheN/C MPA valve 680 may be a two-position digital isolation valve operablebetween a fully open position and a fully closed position. The N/C MPAvalve 680 may be current limited depending on the pressure and flowrates acting on the valve. This provides the advantage of conservingenergy for maintaining the N/C closed valve 680 in an open position whenmaximum current is not required to do so.

When in a closed position, the N/C MPA valve 680 prevents leakage acrossthe boost valve 670 when the boost valve 670 is in an unactuated state.This allows the boost valve 670 to be manufactured with lower toleranceswhich reduces the cost of the boost valve 670. Conventionally, spoolvalves used in boost valves are typically machined to high tolerances tominimize leakage through the boost valve when closed. With the inclusionof the N/C MPA valve 670, the system 500 may include a lower cost boostvalve 670. For example, when the boost valve 670 is in an unactuatedposition, the boost valve 670 isolates the pressurized fluid generatedby the pressure source from the boost conduit 672. Leakage of fluidthrough the spool valve of the boost valve 670 when the boost valve 670is in an unactuated state results in a loss of pressure in the MPA 656.The pump 180 would thereafter intermittently operate to maintain thepressure in the MPA 656 which ultimately consumes energy as used by themotor. When the N/C MPA valve 680 is in a closed position, the boostvalve 670 is isolated from the pressure exerted by the MPA 656 and thepump 652 which could otherwise result in leakage. As a result, the boostvalve 670 may be manufactured at lower tolerances which ultimatelyreduces the cost of the boost valve 670 and greatly minimizes energyconsumption since the motor 654 will not have to operate as often tofill the depleting MPA 656.

The system 500 further includes a first base brake valve 690 and asecond base brake valve 692. The base brake valves 690 and 692 may besolenoid actuated three way valves. The first base brake valve 690 has aport 690 a in fluid communication with the conduit 599 which is in fluidcommunication with the first output chamber 590. A port 690 b is influid communication with the boost conduit 672. A port 690 c is in fluidcommunication with a conduit 700 which is selectively in fluidcommunication with the wheel brakes 506 a and 506 b. The second basebrake valve 692 has a port 692 a in fluid communication with the conduit608 which is in fluid communication with the second output chamber 600.A port 692 b is in fluid communication with the boost conduit 672. Aport 690 e is in fluid communication with a conduit 702 which isselectively in fluid communication with the wheel brakes 506 c and 506d.

The system 500 further includes various valves for permitting controlledbraking operations, such as ABS, traction control, vehicle stabilitycontrol, and regenerative brake blending. A first set of valves includean apply valve 710 and a dump valve 712 in fluid communication with theconduit 700 for cooperatively supplying brake fluid received from theboost valve 670 to the first wheel brake 506 a, and for cooperativelyrelieving pressurized brake fluid from the first wheel brake 506 a to areservoir conduit 711 in fluid communication with the reservoir conduit609. A second set of valves include an apply valve 714 and a dump valve716 in fluid communication with the conduit 700 for cooperativelysupplying brake fluid received from the boost valve 670 to the secondwheel brake 506 b, and for cooperatively relieving pressurized brakefluid from the second wheel brake 506 b to the reservoir conduit 711. Athird set of valves include an apply valve 718 and a dump valve 720 influid communication with the conduit 702 for cooperatively supplyingbrake fluid received from the boost valve 670 to the third wheel brake506 c, and for cooperatively relieving pressurized brake fluid from thethird-wheel brake 506 c to the reservoir conduit 711. A fourth set ofvalves include an apply valve 722 and a dump valve 724 in fluidcommunication with the conduit 702 for cooperatively supplying brakefluid received from the boost valve 670 to the fourth wheel brake 506 d,and for cooperatively relieving pressurized brake fluid from the fourthwheel brake 506 d to the reservoir conduit 711.

The following is a description of the operation of the braking system500. FIG. 10 illustrates the brake system 500 during a normal boostapply operation (a typical braking condition). During boost apply, thebrake pedal 536 is depressed by the driver of the vehicle. The brakepedal 536 is coupled to a travel sensor 713 for producing a signal thatis indicative of the length of travel of the primary piston 526 andproviding the signal to a control module (not shown). The control modulereceives various signals, processes signals, and controls the operationof various components of the brake system 500 in response to thereceived signals. During normal braking operations the pressure source650 and the boost valve 670 are operated to provide boost pressurewithin the conduit 672 for actuation of the wheel brakes 506 a-d. Theconduit 672 provides pressurized fluid to the conduits 700 and 702 viathe energized base brake valves 690 and 692. Under certain drivingconditions, the control module communicates with a powertrain controlmodule (not shown) and other additional braking controllers of thevehicle to provide coordinated braking during advanced braking controlschemes (e.g., anti-lock braking (AB), traction control (TC), vehiclestability control (DISC), and regenerative brake blending).

During a normal boosted braking operation, the flow of pressurized fluidfrom the master cylinder 510 generated by depression of the brake pedalis diverted into the pedal simulator 610. The base brake valves 690 and692 are energized to their positions as shown in FIG. 10 to prevent theflow of fluid from the conduits 599 and 608 through the valves 690 and692. More specifically, movement of the input rod 538 causes the primarypiston 526 to move to the right, as viewing FIGS. 9 and 10. Pressure isbuilt up in the primary chamber 580 and fluid flows into the pressurechamber 618 of the pedal simulator assembly 610 via the conduit 588. Thepressure chamber 618 expands upon continuing movement of the brake pedaland the primary piston 526. Initial movement of the primary piton 526causes rightward movement of the first and second output pistons 530 and532 via the force transmitted through springs 568 and 570. The first andsecond output pistons 530 and 532 will move a relatively small distanceuntil the compensation assemblies 594 and 604 closes off the fluidcommunication between the pressure chambers 590 and 600 to the reservoir512. Further movement of the primary piston 526 will not cause movementof the first and second output pistons 530 and 532 due to the hydrauliclock in the conduits 599 and 608 from the closing of the base brakevalves 690 and 692. Note that fluid flow from the primary chamber 580 tothe reservoir 512 is closed off once a port 553 moves past the seal 552.Thus, the seal 552 may function as a cut-off valve. Prior to movement ofthe primary piston 526, the primary chamber 580 is in fluidcommunication with the reservoir 512 via a conduit 555 and the port 553.

Initial rightward movement of the piston 612 of the pedal simulator 610,as viewing FIG. 10, compresses the spring 626 until the collar 622 restsagainst the piston 612. Note that the spring 628, which has a higherspring rate than the spring 626, may also slightly compress. Continuedmovement of the piston 612 compresses the spring 628. Thus, the pedalforce in the initial stages generally corresponds to the spring rate ofthe spring 626 and the spring 628 in series. Once the collar 622 restsagainst the piston 612, the pedal force generally corresponds to thespring rate of the spring 628. Note that the pedal force is alsodependent upon other springs within the master cylinder 510.

FIG. 11 illustrates the brake system 500 when there is a loss ofelectrical power. In the event of an electrical brake failure, the brakesystem, 500 provides for manual apply or manual push through. During anelectrical failure, the motor 654 might cease to operate, therebyfailing to produce pressurized hydraulic brake fluid from the pump 652.Furthermore, the boost valve 670 might return to a de-energized positionif energized. In this situation, the source of high pressure 650 doesnot supply pressurized fluid and thus the boost valve 670 does notprovide pressurized fluid within the conduit 672. The base brake valves690 and 692 will shuttle to the positions shown in FIG. 11 closing offfluid flow from the conduit 672 to the conduits 700 and 702. In thesepositions, the base brake valves 690 and 692 permit fluid flow from theconduits 599 and 608 to the conduits 700 and 702, respectively. Thus,the master cylinder 510 may now provide manual push through forenergizing the fluid conduits 700 and 702 for actuation of the wheelbrakes 506 a-d.

During manual push through, the first and second output pistons 530 and532 will advance rightward pressurizing the chambers 590 and 600. Upon asufficient distance, the seals 572 and 576 will move past the openingsto the conduits 630, thereby preventing fluid flow from reservoirchamber 620 of the pedal simulator from flowing into the reservoir 512.Thus, the seals 572 and 576 may function as a cut off valve. The fluidwithin the primary chamber 580 is now locked and movement of the primarypiston 526 will cause movement of the first and second output pistons530 and 532.

In another example of a failed condition of the brake system 500, thesource of high pressure 650 may fail as discussed about with respect toFIG. 11 and furthermore one of the pressure chambers 590 or 600 may bereduced to zero or reservoir pressure, such as failure of the seal 574or 578. In this manual push through situation, the pressure within theother non-failed pressure chamber 590 or 600 will be at about twice thepressure if the master cylinder were designed such that each of thesealing areas of the output pistons 630 and 632 (generally at the seal572 and 576) is equal to half of the sealing area of the primary piston526 (generally at the seal 552). In this failed condition, the failedoutput piston 530 or 532 will pull on the primary piston 526 due to thecooperation between the shoulders 560 and 566 and the enlarged heads 556and 562.

There is illustrated in FIG. 12, a detailed cross-sectional view of themaster cylinder 510 which may be used with the system 500. The mastercylinder assembly illustrated in FIG. 12 is similar in structure andfunction as the master cylinder 510 shown in FIG. 8 and thus likenumbers will be used for components having similar functions althoughthe structures may be different. As stated above, the primary piston 526includes a slot 558 and a slot 564 (such as an upper and lower slot) forreceiving enlarged heads 556 and 562 of the first and second outputpistons 530 and 532. Shoulders 560 and 566 retain the enlarged heads 556and 562. For assembly purposes, the enlarged heads 556 and 562 may beinserted into lateral respective slots 558 and 564. Then the combinationof the primary piston 526 and the first and second output pistons 520and 532 may be inserted into the interior of the housing 514. Theprimary piston 526 includes a right-hand most end 515 which abuts thesprings 568 and 570.

In the illustrated embodiment of FIG. 12, the retainer 540 may beretained in the bore 542 of the primary piston 526 by integralcircumferential extending fingers 541 formed on the retainer 540 whichsnap fit against a shoulder 543 formed in the primary piston 526. Thespring 544 may be in the formed of a stack of Belleville washers. Thespring 544 preferably has a higher spring rate than the springs 568 and570 which preferably have a higher spring rate than the springs 596 and606.

FIG. 13 is a graphical representation of a possible pedal force andsimulation pressure vs. pedal travel for various brake systems discussedherein, such as the brake system 500. It is noted that at about 4.2 mmof pedal travel there is a change in slope. This change in slopegenerally corresponds to actuation of the compensation assemblies 594and 604. A change in slope at about 6.3 mm generally corresponds to theclosing of the primary port 533 with seal 552 of the primary piston 526.Another change in slope generally occurs at 24 mm which generallycorresponds to the collar 622 contacting the piston 612 of the pedalsimulator assembly 610. Another change in slope generally occurs atabout 61 mm of travel which generally corresponds to the primary piston526 contacting the first and second output pistons 530 and 532. The lastchange in slope at about 65 mm generally coincides with the bottomingout of the spring washers 544. An advantage of the design of the mastercylinder 510 is that the electronic control module can be configured todetect all or most of the above changes in slope so that specificproblems within the maser cylinder 510 of the brake system 500 can bedetermined if, for example, one or more of the changes in slopes is notdetected. Thus, certain failure modes or operations may be detectableeven though the driver may be unable to determine any problem via pedalfeedback.

There is illustrated in FIG. 14 a cross-sectional view of an embodimentof a MPA 800 (medium pressure accumulator) which may be used in any ofthe brake systems described above. The MPA 800 is mounted in a housing802, such as a housing of a hydraulic control unit or power packsdescribed above. A cup-shaped cover 804 surrounds the components of theMPA 800. The cup-shaped cover 804 may be retained on the housing 802 bya circlip assembly 806. An annular seal 808 seals the cover 804 on thehousing 802. The housing 802 includes a vent conduit 810 which iscovered by a vent cap 812. The vent cap 812 may function as a one waycheck valve permitting the release of air, fluid or other gases from theinterior of the cover 804. The housing 802 further includes a bore 814which receives an end of a piston 816. The end of the piston 816 issealingly engaged with the walls of the bore 814 via seals 818 and 819.The piston 816 is generally fixed relative to the housing 802 duringoperation of the MPA 800.

For description purposes, the MPA 800 will be described as being used asthe MPA 424 in the systems 400 of FIG. 6 but it should be understoodthat the MPA 800 may be used in any suitable brake system such as thosedescribed herein. As such, the housing 802 includes a conduitcorresponding to the conduit 436 in fluid communication with thereservoir 18. The housing 802 includes another conduit corresponding tothe conduit 430 in fluid communication with the priority valve 426. Thepiston 816 includes a central passageway 820 formed therein in fluidcommunication with the conduit 436. The central passageway 820 definesan orifice 822 and a seat 824. A lateral passageway 826 is in fluidcommunication with the central passageway 820 and extends to an outercylindrical surface of the piston 816 between a pair of lip seals 830and 832 mounted in grooves formed in the piston 820. The piston 816further includes a passageway 834 which is in fluid communication withthe conduit 830 through an optional annular filter 832. The other end ofthe piston 816 includes a central bore 836. The seat 824 is between thecentral bore 836 and the central passageway 820.

The MPA further includes a cage 840, a sleeve 842, and a spring 844. Thesleeve 842 has an inverted cup-shape having an inner bore 846 whichreceives the piston 816. The sleeve 842 is sealingly engaged with thelip seals 830 and 832 and is slidably disposed on the piston 816. Thesleeve 842 includes a flange 848. One end of the spring engages the topportion of the cover 804 and the other end of the spring 844 is engagedwith flange 848 to bias the sleeve 842, downwardly as viewing FIG. 14.The sleeve 842 includes a stepped through bore 850 which receives anupper portion of the cage 840. The lower portion of the cage 840 isdisposed in the central bore 836 of the piston 816. An annular seal 852is mounted on the upper portion of the cage 840 and engages the walls ofthe bore 850 of the sleeve 842. The cage 840 includes a stepped throughbore 860 defining a shoulder 862. A pin 870 is disposed in the bore 860.One end of the pin 870 includes a valve member 872 in the shape of aspherical end which engages with the seat 824 of the piston 816. Theother end of the pin 870 includes an outwardly extending flange 874. Thepin 870 may be retained in the bore 860 by a press fit ball 876.

In operation, a pressure chamber 880 defined between the piston 816 andthe sleeve 842 expands as the sleeve 842 is moved upward by the entranceof fluid via the conduit 420 and the passageway 834. The cage 840 moveswith the sleeve 842. The cage 840 is connected to the sleeve 842 via thefrictional engagement of the seal 852. Movement of the sleeve 842compresses the spring 844, thereby generating a pressure within thechamber 880. The MPA 800 preferably maintains a pressure within adesired range, for example, between 30-80 bar. Alternatively, the MPA800 may have a different operating pressure range. The MPA 800 includestwo bypass functions which vents the chamber 880 to the reservoir whenone or more predetermined pressure levels are reached within the chamber880. The first or main bypass function (valve) occurs when the sleeve842 is moved sufficiently upward, as viewing FIG. 14, such that theshoulder 862 of the cage 840 abuts the flange 874 of the pin 870 liftingthe pin 870. Lifting of the pin 870 causes the valve member 872 to liftfrom the seat 824 permitting the flow of fluid from the chamber 880 tothe central passageway 820 and the conduit 436 which is in fluidcommunication with the reservoir. This bypass function maintains thechamber 880 at a predetermined pressure level.

The MPA 800 includes an auxiliary bypass function to prevent a pressurebuild up in the MPA 800 in case the first bypass function fails. Forexample, if the pin 870 were to break and maintain the valve member 872against the valve seat 824, an undesirable pressure may build up withinthe MPA 800. In this situation, the sleeve 842 advances further upwarduntil the end of the cage 840 hits the roof of the cover 804. Furthermovement causes the cage 840 to dislodge from the sleeve 842 at the seal852, thereby providing a flow path from the chamber 880 to the interiorof the cover 804 where the spring 844 is housed. This fluid can then bevented through the vent 810. This auxiliary bypass function helpsprevent the pressure from building up and causing the cover 804 to bedislodged from the housing 802.

There is illustrated in FIG. 15 a cross-sectional view of an embodimentof a base brake valve 900 which may be used in any of the brake systemsdescribed above. For description purposes, the base brake valve 900 willbe described as being used as the base brake valve 900 in the systems 10of FIG. 1 but it should be understood that the base brake valve 900 maybe used in any suitable brake system such as those described herein. Thebase brake valve 900 is a solenoid actuated three-way valve. The valve900 is mounted in a bore 901 of a housing 902, such as the block of thehydraulic control unit 12, and is in fluid communication with theconduits 260, 324, and 156. The valve 900 includes a body 904 retainedin the housing 902. The body 904 includes a stepped central bore 906defining a first valve seat 908. The body 904 includes a pair ofpassageways 910 and 912 extending through the central bore 906 and arein fluid communication with the conduits 260 and 324, respectively. Thevalve 900 includes an armature 920 that moves downwardly, as viewingFIG. 15, when the valve 900 is energized. Movement of the armature 920causes a pin 922 to lift a ball 922 from the first valve seat 908. Aspring 924 biases the ball 922 against the seat 908. Further movement ofthe pin 922 will push the ball 922 against a second valve seat 926 of aretainer 928. The retainer 928 includes a through bore in fluidcommunication with the conduit 156. When the ball 922 is seated on thefirst valve seat 908, the valve 900 prevents the flow of fluid at theconduit 260 but permits fluid flow between the conduits 156 and 324.When the ball 922 is seated on the second valve seat 926, the valve 900prevents the flow of fluid at the conduit 156, but permits the flow offluid between the 260 and 324. The valve 900 can include a combinationfiler and sealing member 930. The retainer 928 may include a restrictedorifice 932 to restrict the flow of fluid therethrough.

There is illustrated in FIG. 16 a cross-sectional view of an embodimentof a simulation valve 1050 which may be used in any of the brake systemsdescribed above. The simulation valve 1050 is received in a bore 1005formed in the housing 1002. The simulation valve 1050 includes a sleeve1200 having a first end 1202 and a second end 1204 and defining an axisB. An armature 1206 has a first end 1208 and a second end 1210 and isslidably received in the sleeve 1200. The simulation valve 1050 furtherincludes a coil assembly (not shown) disposed about the sleeve 1200.

In the illustrated embodiment, the sleeve 1200 is formed as a singlepiece from ferromagnetic material in a deep drawing process. An exampleof suitable ferromagnetic material is low-carbon steel. It will beunderstood however, that low-carbon steel is not required, and that thesleeve 1200 may be formed from any other desired ferromagnetic material.

The sleeve 1200 includes a first body portion 1212 having a firstdiameter, a second body portion 1214 having a second diameter, and athird body portion 1216 having a third diameter. The second end 1204 ofthe sleeve 1200 includes a radially inwardly extending first shoulder1218 extending between the second body portion 1214 and the third bodyportion 1216, and defining a valve seat 1218. A magnetic core 1220 isattached to the first end of the sleeve 1200, thereby closing the firstend. 1202 of the sleeve 1200. The core 1220 may be attached to the firstend 1202 of the sleeve 1200 by any suitable means, such as with a singlelaser weld. Alternatively, the core 1220 may be attached to the firstend 1202 of the sleeve 1200 by any other desired method. A plurality offluid passages 1205 are formed in the sleeve 1200.

The armature 1206 is slidably received in the sleeve 1200. In theexemplary embodiment illustrated, the first end 1208 of the armature1206 includes a spring cavity 1222. The first spring 1224 is disposed inthe cavity 1222 and engages the armature 1206 and the core 1220 to urgethe armature 1206 and the poppet 1226 (described in detail below) towardthe valve seat 1218 when the simulation valve 1050 is in the closedposition. When the coil assembly is energized, the armature 1206 and thepoppet 1226 are disposed at an extreme of travel away from the valveseat 1218, such that the simulation valve 1050 is in an open position(not shown).

A recess 1228 is formed in an end surface of the second end 1210 of thearmature 1206. A spherical valve part or ball 1230 is pressed into therecess 1228. In the illustrated embodiment, the ball 1230 is formed fromsteel. Alternatively, the ball 1230 may be formed from any othersubstantially non-deformable metal or non-metal. In the illustratedembodiment, the armature 1206 is formed from ferromagnetic material in acold forming process. An example of suitable ferromagnetic material islow-carbon steel. It will be understood however, that low-carbon steelis not required, and that the armature 1206 may be formed from any otherdesired ferromagnetic material.

The poppet 1226 is disposed between the armature 1206 and the valve seat1218 and includes a generally cylindrical body 1232 having a first end1234, a second end 1236, and a bore 1238 therethrough. The first end1234 defines a seat portion 1240. A radially outwardly extendingcircumferential shoulder 1242 is defined in an outer surface of thepoppet 1226 intermediate the first end 1234 and the second end 1236. Asecond spring 1244 extends between the second end 1210 of the armature1206 and the shoulder 1242. A radially outwardly extendingcircumferential flange 1245 is also formed in an outer surface of thepoppet 1226.

In the illustrated embodiment, the poppet 1226 is formed as a singlepiece from plastic material. An example of suitable plastic material isnylon. It will be understood however, that nylon is not required, andthat the poppet 1226 may be formed from any other desired material.

A substantially cup-shaped cage 1246 includes a first end 1248 and asecond end 1250. The second end 1250 of the cage 1246 includes aradially inwardly extending shoulder 1252 defining a cage opening 1254.A plurality of fluid passages 1255 are formed in the cage 1246. In theillustrated embodiment, the cage 1246 is formed as a single piece fromferromagnetic material in a deep drawing process. An example of suitableferromagnetic material is low-carbon steel. It will be understoodhowever, that low-carbon steel is not required, and that the cage 1246may be formed from any other desired ferromagnetic material.

The flange 1245 of the poppet 1226 is slidably received within the cage1246. The second end 1236 of the poppet 1226 extends through the opening1254 of the cage 1246 arid further sealingly engages the valve seat1218.

An electrical coil (not shown) is disposed about the sleeve 1200,armature 1206, and magnetic core 1220 and selectively induces a magneticflux in the armature 1206. Because the simulation valve 1050 is anormally closed valve, the first spring 1224 urges the armature 1206 andthe poppet 1226 into contact with the valve seat 1218 when the coilassembly of the simulation valve 1050 is not energized, thereby blockingfluid flow through the simulation valve 1050. When the coil assembly isenergized, the armature 1206 and the poppet are urged away from thevalve seat 218 to permit fluid flow through the simulation valve 1050.

A circumferentially extending internal band filter 1256 includes a firstend 1258 and a second end 1260 and may be placed about the second bodyportion 1214 of the sleeve 1200. In the illustrated embodiment, thesecond end 1260 includes an opening 1261 through which the third bodyportion 1216 extends. The second end 1260 of the filter 1256 furtherengages the first shoulder 1218. It will be understood however, thatsuch a band filter 1256 is not required. A lip seal 1262 is disposedabout the third body portion 1216 of the poppet 1226 between the filter1256 and the second end 1236 of the poppet 1226.

The ball 1230 acts as a valve sealing element and engages the seatportion 1240 of the poppet 1226 when the valve 1050 is in the closedposition (e.g. when the coil assembly is not energized).

If a pressure difference between the inlet side (see the arrow 1264) andthe outlet side (see the arrow 1266) of the housing 1002 is relativelysmall, and if the closing force acting on the poppet 1226 is lower thanthe force exerted by the second spring 1244, then the valve seat 1218 isopened without movement of the poppet 1226 relative to the armature1206.

When the pressure difference between the inlet side and the outlet side1266 of the housing 1002 is relatively large, then the hydraulic closingforce acting on the poppet 1226 may be greater than the force exerted bythe second spring 1244.

The magnetic force, which is low at the beginning of the armature 1206stroke (upward as viewed in FIG. 16), will, upon movement of thearmature 1206 toward the core 1220, overcome the pre-stressing force ofthe first spring 1224 and the second spring 1244 as well as thehydraulic closing force acting on the armature 1206, in order to openthe poppet 1226 (by moving the ball 1230 of the armature 1206 away fromthe seat portion 1240 of the poppet 1226).

Opening the seat portion 1240 of the poppet 1226 over the course ofarmature 1206 movement, fluid may flow through the bore 1238 of thepoppet 1226 to the outlet side 266. As a result of the opening of theseat portion 1240, the pressure difference is reduced and the closingforce acting upon the poppet 1226 is reduced.

There is illustrated in FIG. 17 a portion of an alternate embodiment ofa brake pedal unit (BPU), indicated generally at 1320. The brake pedalunit 1320 is similar in structure and function as the brake pedal unit20 illustrated in FIGS. 2-4. Differences between the brake pedal units20 and 1320 will be described below. The brake pedal unit 1320 may beconnected to a reservoir, such as the reservoir 18. Although the portionof the brake pedal unit 1320 which is connected to the reservoir 18 isnot illustrated in FIG. 17, it should be understood that the structureand function of that portion (right-hand end as viewing FIG. 17) may beformed similarly as the brake pedal unit 20 illustrated in FIGS. 2-4.

The brake pedal unit 1320 includes a housing 1324 having various boresformed therein for slidably receiving various cylindrical pistons andother components therein. In the embodiment illustrated in FIG. 17, thehousing 1324 is formed from a first portion 1324 a and a second portion1324 b. The first and second portions 1324 a and 1324 b may be coupledtogether by any suitable manner, such as by mounting bolts. The housing1324 generally includes a first bore 1326, a second bore 1328, a thirdbore 1330, and a fourth bore 1332. The first and second bores 1326 and1328 are formed in the first portion 1324 a of the housing 1324 and areaxially aligned with one another. The second bore 1328 has a largerdiameter than the first bore 1326. The third and fourth bores 1330 and1332 are formed in the second portion 1324 b and are in communicationwith the larger diameter second bore 1328. The third bore 1330 islocated laterally (or above when viewing FIG. 17) with respect to thefourth bore 1332.

The brake pedal unit 1320 further includes an input piston 1334, anintermediate piston 1336, a first output piston 1338, and a secondoutput piston 1340. The input piston 1334 is slidably disposed in thefirst bore 1326. The intermediate piston 1336 is slidably disposed inthe second bore 1328. The first output piston 1338 is slidably disposedin the third bore 1330. The second output piston 1340 is slidablydisposed in the fourth bore 1332. Although not shown in their entiretyin FIG. 17, the first and second output pistons 1338 and 1340 may havethe same structure as the output pistons 38 and 40 of the brake pedalunit 20 and may function in a similar manner.

A brake pedal (not shown) is coupled to a first end 1344 of the inputpiston 1334 via an input rod 1345 and a coupler 1346. The coupler 1346is threadably engaged with the first end 1344 of the input piston 1334and can include a socket for receiving a spherical end of the input rod1344. The input piston 1334 includes an enlarged second end 1352 thatdefines a shoulder 1354. In the rest position shown in FIG. 17, theshoulder 1354 of the input piston is spaced slightly away from ashoulder 1356 formed between the first and second bores 1326 and 1328 ofthe housing 1324. An outer surface 1357 of the input piston 1334 isengaged with lip seals 1358 and 1360 mounted in grooves formed in thehousing 1324. One of the differences between the brake pedal units 20and 1320 is that the input piston 1334 of the brake pedal unit 1320includes first and second frustoconical surfaces 1361 and 1363. As bestshown in FIG. 18, the first frustoconical surface 1361 extends along alength X of the input piston 1334. The second frustoconical surface 1363extends along a length Y of the input piston 1334. For reasons whichwill be discussed below, the first and second frustoconical surfaces1361 and 1363 are angled or sloped relative to a central axis Z definedalong the length of the input piston 1334. During operation of the brakepedal unit 1320, the input piston 1334 moves along the axis Z. Movementof the input piston 1334 causes the first and second frustoconicalsurfaces 1361 and 1363 to slide along an elastomeric seal 1365. The seal1365 is housed in a groove 1367 formed in a wall of the first bore 1326.For reasons which will be discussed further below, the seal 1365 isexpanded outwardly as the input piston 1334 moves in a rightwarddirection, as viewing FIGS. 17 and 18, due to the inclined first andsecond frustoconical surfaces 1361 and 1363.

As shown in FIGS. 17 and 18, the input piston 1334 includes an angledshoulder 1369 located at the right-hand end of the second frustoconicalsurface 1363 and connects with the outer cylindrical surface 1357. Whenthe brake pedal unit 1320 is in a “rest” position (i.e., the pedal hasnot been depressed by the driver of the vehicle), the shoulder 1369abuts the seal 1365 and prevents further leftward movement of the inputpiston 1334. Unlike the brake pedal unit 20, the brake pedal unit 1320is configured such that the abutment of the seat 1365 and the angledshoulder 1369 provides a gap between the shoulder 1354 of the inputpiston 1334 and the shoulder 1356 of the housing 1324. Thus, duringrelease of the brake pedal by the driver, the input piston 1334 will becushioned to its rest position by the abutment of the elastomeric seal1365 and the angled shoulder 1369 as compared to a metal to metalcontact of the brake pedal unit 20. Thus, the elastomeric seal 1365functions as a deformable cushion or stop to dampen travel of the inputpiston 1334 to its rest position. The seal 1365 may also help avoid asevere step or increase in force to start movement of the brake pedal bythe driver. Of course, the shoulder 1354 will still provide a stop forleftward movement of the input piston 1334 such as in cases of failureof the seal 1365. Although the seal 1365 is shown in the form of anO-ring having a generally circular cross-sectional shape, it should beunderstood that the seal 1365 can have any suitable shape. For example,the seal 1365 could be a quad ring-type seal. It should also beunderstood that the shoulder 1369 need not be angled as shown in FIGS.17 and 18, but could have any suitable shape to permit the desiredabutment and cushioning properties in cooperation with a suitably shapedseal 1365.

The input piston 34 includes a central bore 1362 formed through thesecond end 52. One or more lateral passageways 1364 are formed throughthe input piston 1334. The lateral passageways 1364 extend from thecentral bore 1362 to the outer surface 1357 of the input piston 1334. Asshown in FIG. 18, the lateral passageways 1364 may pass through thefirst frustoconical surface 1361. In one embodiment, the input piston1334 includes four lateral passageways 1364 spaced 90 degrees about thecircumference of the input piston 1334.

The brake pedal unit 1320 is in the rest position as shown in FIGS. 17and 18. In the rest position, the passageways 1364 of the input piston1334 are between the lip seals 1358 and 1360. In this position, thepassageways 1364 are in fluid communication with a conduit 1366 formedthrough the first portion 1324 a of the housing 1324. The conduit 1366is in fluid communication with the reservoir in a similar manner as thebrake pedal unit 20. In the rest position, the passageways 1364 are alsoin fluid communication with a conduit 1372 formed in the housing 1324which leads to a cut off valve or simulation valve 1374. The simulationvalve 1374 may be electrically operated. The simulation valve 1374 maybe operated in a similar manner as the simulation valve 74 describedabove.

The intermediate piston 1336 is slidably disposed in the second bore1328 of the housing 1324. The outer wall of the intermediate piston 1336is engaged with a lip seal 1380 mounted in a groove formed in thehousing 1324. The intermediate piston 1336 includes a first end 1382having a cavity 1384 formed therein. A second end 1386 of theintermediate piston 1336 includes a cavity 1388 formed therein. Thecavity 1388 includes a floor 1390 defining a pivot groove 1392 whichfunctions as a “rocker-like” or “cam-like” support surface for a wobbleplate 1394.

The central bore 1362 of the input piston 1334 and the cavity 1384 ofthe intermediate piston 1336 house various components defining a pedalsimulator, indicated generally at 1400. The pedal simulator 1400 may besimilar in structure and function as the pedal simulator 100 of thebrake pedal unit 20. The pedal simulator includes a caged springassembly, indicated generally at 1402, and is defined by a pin 1404, aretainer 1406, and a low rate simulator spring 1408. A stop 1423 isprovided which may be integrally formed with the retainer 1420 or may bea separate piece attached thereto. The stop 1423 is axially aligned withthe pin 1404. The stop 1423 includes a plurality of disc springs 1418(also known as Belleville washers, coned-disc springs, or cupped springwashers) which may be in a preloaded state or may mounted in anon-loaded state.

The pedal simulator 1400 further includes a high rate simulator spring1430 which is disposed about the pin 1404. The terms low rate and highrate are used for description purposes and are not intended to belimiting. It should be understood that that the various springs of thepedal simulator 1400 may have any suitable spring coefficient or springrate. In the illustrated embodiment, the high rate simulator spring 1430preferably has a higher spring rate than the low rate simulator spring1408. A separator member or sleeve 1432 is slidably disposed in thecentral bore 1362 of the input piston 1334 and includes a through borethrough which the pin 1404 extends. One end of the high rate simulatorspring 1430 engages with the sleeve 1432. The other end of the high ratesimulator spring 1430 is shown in FIG. 17 in a non-engaged position andspaced away from an end of the retainer 1406. A blend spring 1440 isdisposed about the pin 1404. One end of the blend spring 1440 engageswith the sleeve 1432, and the other end of the blend spring 1440 engageswith a bottom floor portion of the central bore 1362 of the input piston1334. The housing 1324, the input piston 1334 (and its seals), and theintermediate piston 1336 (and its seals) generally define a simulationchamber 1444. The simulation chamber 1444 is in fluid communication witha conduit 1446 which is in fluid communication with the simulation valve1374. The conduit 1446 includes a reduced diameter fixed orifice 1447therein. In the embodiment shown in FIG. 17, the fixed orifice 1447 isformed in a plug 1449 that is threaded or otherwise mounted in thehousing 1324. The fixed orifice 1447, or a plurality of fixed orifices,may be formed in the conduit 1446 by any suitable manner. The fixedorifice 1447 may be located vertically above the simulation chamber 1444to provide for ease of bleeding fluid to the simulation chamber 1444. Anauxiliary conduit 1451 is in fluid communication between the conduit1446 and to the simulation chamber 1444. The auxiliary conduit 1451ports into the first bore 1326 at an orifice 1453. In the embodimentshown, the orifice 1453 is larger in cross-sectional area compared tothe fixed orifice 1447. It should be understood that the orifices 1447and 1453 may be sized to any suitable cross-sectional area. As will bediscussed below, the flow rate through the orifice 1453 also depends onthe location of the input piston 1334 due to the first and secondfrustoconical surfaces 1361 and 1363 in proximity to the opening of theorifice 1453 at the wall of the first bore 1326.

The operation of the brake pedal unit 1320 will now be described. Asstated above, the brake pedal unit 1320 may operate in a similar manneras the brake pedal unit 20 of FIGS. 2-4 and will be described as beingused in cooperation with the brake system 10 of FIG. 1. Differences inthe operation between the brake pedal units 1320 and 20 will bediscussed below. Similar functions and operations may not necessarily beduplicated in this discussion of the operation of the brake pedal unit1320. FIGS. 17 and 18 illustrate the brake pedal unit 1320 in its restposition. In this condition, the driver is not depressing the brakepedal 42. Also in the condition, the simulation valve 74 may beenergized or not energized. During a typical braking condition, thebrake pedal 42 is depressed by the driver of the vehicle. The brakepedal 42 is coupled to the travel sensor 240 for producing a signal thatis indicative of the length of travel of the input piston 34 andproviding the signal to an electronic control module (not shown). Duringa normal boost apply braking operation, the flow of pressurized fluidfrom the brake pedal unit 1320 generated by depression of the brakepedal 42 is diverted into the pedal simulator assembly 1400. Thesimulation valve 1374 is actuated to divert fluid through the simulationvalve 1374 from the simulation chamber 1444 to the reservoir. Note thatfluid flow from the simulation chamber 1444 to the reservoir 18 isclosed off once the passageways 1364 move past the seal 1360. Prior tomovement of the input piston 1334, as shown in FIG. 17, the simulationchamber 1444 is in fluid communication with the reservoir 18. Movementof the input rod 1345 by the pedal 42 causes movement of the inputpiston 1334 in a rightward direction, as viewing FIGS. 17 and 18.Initial movement of the input piston 34 causes movement of theintermediate piston 36 via the low rate simulator spring 108 and willclose the center valve assemblies associated with the first and secondoutput pistons 1338 and 1340, thereby preventing further movement of thepistons 1338 and 1340. As the input piston 1334 continues to moverightward, as viewing FIGS. 17 and 18, the various springs of the pedalsimulator assembly 1400 are compressed, thereby providing a feedbackforce to the driver of the vehicle.

During rightward movement of the input piston 1334 relative to agenerally stationary intermediate piston 1336, such as during a normalbraking operation in which the simulation chamber 1444 is nothydraulically locked, the seal 1365 is moved radially outwardly into thegroove 1367. During this rightward movement of the input piston 1334,radially outwardly extending forces are acting on the seal 1365 causingit to be expanded or stretched yet confined in the groove 1367. Thisdeformation and expansion results in an increase in frictional forcesduring movement of the input piston 1334 caused by the reactionarycompressive forces of the seal 1365 acting against the frustoconicalsurfaces 1361 and 1363. Due to the frustoconical shape of the surfaces1361 and 1363 of the input piston 1334, the frictional forces increaseas the input piston 1334 moves rightwardly as viewing FIGS. 17 and 18.Thus, as the input piston 1334 is advanced in a rightward direction asviewing FIGS. 17 and 18, the rate of friction is progressive orincreases the farther the input piston is advanced. The cross-sectionalprofile or slopes of the frustoconical surfaces 1361 and 1363 can beconfigured or shaped to provide a desired progressive hysteresis suchthat there is increased friction with an increase in travel of the inputpiston 1334. For example, the angle or slope of the frustoconicalsurfaces 1361 and 1363 may be configured to mimic the “pedal feel” of aconventional vacuum boosted system. Although the embodiment of the inputpiston 1334 has two frustoconical surfaces of different slope angles,the profile of the input piston 1334 can be formed into any suitableshape to provide a desired feedback force. For example, thefrustoconical surfaces 1361 and 1363 need not be linear (in across-sectional profile), as shown in FIGS. 17 and 18, but can have acurvilinear shape. However, a curvilinear frustoconical shape may bemore difficult and expensive to manufacture so a single or multiplelinear sloped frustoconical surface may be sufficient to achieve adesired force profile. In the illustrated embodiment, this isaccomplished by having two linear slopes wherein the first frustoconicalsurface 1361 is slightly angled or sloped relative to the outercylindrical surface 1357 and the second frustoconical surface 1363 has agreater angle of slope than the first frustoconical surface 1361.

If desired, the brake pedal unit 1320 may be configured to provide foran initial movement of the input piston 1334 such that the seal 1365does not engage yet with the second frustoconical surface 1363. Theconfiguration and size of the seal 1365, the groove 1367, the secondfrustoconical surface 1363, and the angled shoulder 1369 may be designedto permit a 2 mm amount of travel, for example, prior to engagementbetween the seal 1365 and the second frustoconical surface 1363. Theconfiguration may be made, for example, to mimic the reactionary forcesor pedal feel of a conventional vacuum assisted brake system.

The brake pedal unit 1320 also provides a damping effect during a normalboosted brake operation. For example, in a spike apply in which thedriver presses on the brake pedal 42 in a rapid and forceful manner, thefixed orifice 1447 restricts the flow of fluid between the simulationchamber 1444 and the reservoir via the conduit 1446, thereby impedingadvancement of the input piston 1334. Additionally, the orifice 1453provides additional or secondary damping as fluid flows between thesimulation chamber 1444 and the reservoir. However, this additional orsecondary damping provides progressive damping in that the amount ofdamping is increased with an increase in pedal travel because a port oropening 1455 of the orifice 1453 to the first bore 1326 is being chokedoff by the sloping sides of the frustoconical surfaces 1361 and 1363. Asthe input piston 1334 continues to move rightward as viewing FIGS. 17and 18, the frustoconical surfaces 1361 and 1363 become closer to theopening 1455 of the orifice 1453, thereby further restricting the flowof fluid therethrough. The sizes and dimensions of the orifice 1453 andthe frustoconical surfaces 1361 and 1363 may be tuned or configured toprovide a desired progressive damping profile.

Another difference between the brake pedal units 20 and 1320 is that thebrake pedal unit 1320 does not include a check valve assembly 320 as inthe brake pedal unit 20 of FIGS. 2-4. The check valve assembly 320provided a bypass from the reservoir to the simulation valve 74. Thecheck valve assembly 320 may be eliminated since the spacing between thesecond frustoconical surface 1363 and the orifice 1453 is relativelylarge during the initial stages of movement of the input piston 1334.

Although the term “seal” is used for the O-ring type structureidentified by reference number 1365, it should be understood that theseal 1365 need not have to provide sealing properties for structureswithin the brake pedal unit 1320. For example, the seal 1365 could beconfigured as a different elastomeric or spring like device located in adifferent portion of the brake pedal unit 1320 while still providingprogressive force and/or damping properties during travel of the inputpiston 1334.

There is illustrated in FIG. 19 an alternate embodiment of a vehiclebrake system, indicated generally at 1500. Similar to the abovedescribed brake systems, the brake system 1500 may suitably be used on aground vehicle such as an automotive vehicle having four wheels and awheel brake for each wheel. Furthermore, the brake system 1500 can beprovided with other braking functions such as anti-lock braking (ABS),other slip control features, and regenerative braking blending toeffectively brake the vehicle. The brake system 1500 is similar infunction and structure of some aspects of the above described brakesystems, such as for example the brake system 460 illustrated in FIG. 7.

The brake system 1500 generally includes a hydraulic control unit 1512and a block or power pack assembly 1514. The hydraulic control unit 1512includes a brake pedal unit 1520 which may be similar in structure andfunction as the brake pedal unit 20 described above (or any of the otherbrake pedal units described herein). The hydraulic control unit 1512includes a simulation valve 1574 which may be similar in structure andfunction as the simulation valve 74 described above.

One of the differences between the systems 460 and 1500 is that thebrake system 1500 includes a different pump assembly 1530. Similar tothe pump assembly 262 described above, the pump assembly 1530 mayinclude a plurality of pistons, such as six, as shown in FIG. 97. Thepump assembly 1530 may be configured into a pair of three pistonsub-assemblies 1530 a and 1530 b, having either one or two motors 1532connected thereto. The sub-assemblies 1530 a and 1530 b can beconfigured completely separate from one another with a separate motordriving three pistons, or may be configured such that a single motordrives all six pistons, for example. The pistons can be the same size orthe pistons from one sub-assembly may be sized differently from theother sub-assembly such that different fluid output is provided. In theembodiment illustrated in FIG. 19, the pistons of the sub-assembly 1530a may be smaller in diameter than the pistons of the sub-assembly 1530b.

The brake circuit 1500 includes pump outlet conduits 1570 and 1572. Theconduit 1570 is in fluid communication with the pump outlet of the subassembly 1530 a and a pair of base brake valves 1536 and 1538. The basebrake valves 1536 and 1538 are actuated to control the fluid pressure inconduits 1540 and 1542 to the wheel brakes 1516 a-1516 d in a similarmanner as the brake system 10 in FIG. 1. For example, the base brakevalves 1536 and 1538 may be actuated to their positions shown in FIG. 19to permit the flow of fluid from the brake pedal unit 1520, such asduring manual push through operation. During normal boosted braking, thebase brake valves 1536 and 1538 may be actuated to permit the flow offluid from the conduit 1570. The conduit 1572 is in fluid communicationwith wheel brakes 1516 a and 1516 b via conduits 1544 and 1546. A checkvalve 1550 prevents fluid flow in the direction from the conduit 1542into the conduit 1572. A check valve 1552 prevents fluid flow in thedirection from the conduit 1540 into the conduit 1572.

The brake circuit 1500 includes a first boost valve 1560 in fluidcommunication with the conduit 1570 and a reservoir conduit 1561. Thereservoir conduit 1561 is in fluid communication with the pump inletsand the reservoir 1518 connected to the brake pedal unit 1520. The firstboost valve 1560 may be a proportionally controlled solenoid valve. Thefirst boost valve 1560 may be actuated to control the pressure in theconduit 1570 from the outlet of the pump sub-assembly 1530 a such as bymoving between open and closed positions to selectively vent fluid tothe reservoir conduit 1561.

The brake circuit 1500 includes a second boost valve 1562 in fluidcommunication with the conduit 1572 and the reservoir conduit 1561. Theboost valve 1560 may be a proportionally controlled solenoid valve. Thesecond boost valve 1562 may be actuated to control the pressure in theconduit 1572 from the outlet of the pump sub-assembly 1530 b such as bymoving between open and closed positions to selectively vent fluid tothe reservoir conduit 1561.

During operation of the brake system such as in a normal boosted brakeapply, the pump assembly 1530 may be actuated in a variety of ways. Forexample, the pump assembly 1530 may be operated in an energy saving modein a similar manner as the pump assembly 262 of FIG. 1 such that thesecond boost valve 1562 is moved to its open position so that fluid flowfrom the outlet of the pump sub-assembly 1530 b loops in a relativelylow load mode between the conduits 1572 and 1561. Alternatively, bothpump assemblies 1530 a and 1530 b may be operated simultaneously withthe boost valves 1560 and 1562 in their closed positions for quietoperation of the pump assembly 1530 during low pressures. Similar to thebrake system 10 of FIG. 1, the apply and dump valves are controlled toprovide the desired pressure level at the wheel brakes 1516 a-d.

The brake system 1500 may include various pressure sensors, as shown inFIG. 19, for monitoring the pressure levels in various conduits. Thebrake system 1500 may also be configured to run diagnostics to determineif various components of the brake system 1500 are operating properly.For example, diagnostics could be run to determine if the check valves1550 and 1552 are operating properly so that any fluid leaks aredetected therethrough.

There is illustrated in FIG. 20 an alternate embodiment of a vehiclebrake system, indicated generally at 1600. Similar to the abovedescribed brake systems, the brake system 1600 may suitably be used on aground vehicle such as an automotive vehicle having four wheels and awheel brake for each wheel. Furthermore, the brake system 1600 can beprovided with other braking functions such as anti-lock braking (ABS),other slip control features, and regenerative braking blending toeffectively brake the vehicle. The brake system 1600 is similar infunction and structure of some aspects of the above described brakesystems, such as for example the brake system 1500 illustrated in FIG.19.

The brake system 1600 generally includes a hydraulic control unit 1612and a block or power pack assembly 1614. The power pack assembly 1614 issimilar in structure and function as the power pack assembly 1514described above with respect to FIG. 19. One of the differences betweenthe brake systems 1500 and 1600 is that the brake system 1600 includes adifferent type of brake pedal unit 1620. A more detailed illustration ofthe brake pedal unit 1620 is shown in FIG. 21. The hydraulic controlunit 1612 includes a simulation valve 1674 which may be similar instructure and function as the simulation valve 74 described above.

The brake pedal unit 1620 includes a pedal simulator, indicatedgenerally at 1621, which is similar in structure and function as thepedal simulator 100 of the brake pedal unit 20 shown in FIGS. 1 and 2and the brake pedal unit 1320 shown in FIGS. 19 and 20. The pedalsimulator 1621 includes a simulation chamber 1644 which is in selectivefluid communication with a reservoir 1618 via a simulation valve 1674.The pedal simulator 1620 and simulation valve 1674 are operated in asimilar manner as the pedal simulators 100, 1400 and simulation valves74, 1374 described above with the respect to the brake systems 20, 1320.One of the main differences between the brake pedal units 1320 and 1620is that the brake pedal unit 1620 includes a pair of axially alignednested output pistons 1638 and 1640 instead of the pair of radiallyspaced apart parallel pistons 1338 and 1340 of the brake pedal unit1320.

Similar to the brake pedal unit 1320, the brake pedal unit 1620 includesa housing 1624, an input rod 1645, a coupler 1646, an input piston 1634,a caged spring assembly 1702 (pin 1704, retainer 1706, low ratesimulator spring 1708), and a high rate simulator spring 1730. Theleft-hand end of the first output piston 1638 cooperates with the pedalsimulator assembly 1621 in a similar manner as the brake pedal units1320.

Although the first and second output pistons 1638 and 1640 areconfigured differently in the housing 1624 compared to the outputpistons of the brake pedal unit 1320 shown in FIGS. 17 and 18, the firstand second output pistons 1638 and 1640 operate to provide pressurizedfluid to conduits 1756 and 1766, respectively, such as during a manualpush through or a manual apply brake operation. The conduits 1756 and1766 are in fluid communication with the base brake valves of the powerpack 1614. More specifically, rightward movement of the first outputpiston 1638 (as viewing FIG. 20) pressurizes a first output pressurechamber 1798 which is in fluid communication with the conduit 1756.Rightward movement of the second output piston 1640 (as viewing FIG. 20)pressurizes a second output pressure chamber 1828.

A return spring 1639 biases the first output piston 1638 in a leftwarddirection as viewing FIGS. 20 and 2. A separator spring 1641 biases thefirst and second output pistons 1638 and 1640 away from each other. Theseparator spring 1641 is preferably a lighter spring than the returnspring 1639 such that the return spring 1639 has a greater spring forcethan the separator spring 1641.

The first and second output pistons 1638 and 1640 may be mechanicallyconnected together such that there is limited play or movement betweenthe pistons 1638 and 1640. This type of connection permits the first andsecond output pistons 1638 and 1640 to move relative to one another byrelatively small increments to compensate for pressure and/or volumedifferences in their respective output circuits. However, under certainfailure modes it is desirable that the second output piston 1640 isconnected to the first output piston 1638. For example, if the brakesystem 1600 is under a manual push through mode and additionally fluidpressure is lost in the output circuit relative to the second outputpiston 1640, such as for example, in the conduit 1766, the second outputpiston 1640 will be forced or biased in the rightward direction due tothe pressure within the first output pressure chamber 1798. If the firstand second output pistons were not connected together, the second outputpiston 1640 would freely travel to its further most right-hand position,as viewing FIG. 20, and the driver would have to depress the pedal adistance to compensate for this loss in travel. However, because thefirst and second pistons 1638 and 1640 are connected together, thesecond output piston 1640 is prevented from this movement and no loss oftravel occurs in this type of failure.

In the embodiment shown in FIG. 21, the second output piston 1640includes a slot 1643 which receives a pin 1647 fixed to the first outputpiston 1638. Since the width or diameter of the pin 1647 is less thanthe axial length of the slot 1643, the first and second output pistons1638 and 1640 can move relative to one another by a distancecorresponding to the remaining length of the slot 1643. Alternatively,the pistons 1638 and 1640 may be configured such that the right-hand endof the first output piston 1638 will abut against a shoulder 1651 formedin the second output piston 1640 prior to the pin 1647 bottoming out onthe right-hand end of the slot 1643. In this manner, forces will betransmitted through the shoulder 1651 instead of the pin 1647 when thefirst output piston 1638 pushes against the second output piston 1640such as during a failure in the second output circuit.

During a rest mode when the input piston 1634, the first output piston1638, and the second output piston 1640 are in the leftmost position asviewing FIG. 21, their pressure chambers 1644, 1798, and 1828 are influid communication with the reservoir 1618. Sufficient rightwardmovement of the pistons eventually closes off communication with thereservoir 1618. In particular, the input piston 1634 includes radiallyextending compensation ports 1664 which when moved past a seal 1660closes off fluid communication with the reservoir 1618 via a conduit1665. The first output piston 1638 includes radially extendingcompensation ports 1670 which when moved past a seal 1671 closes offfluid communication with the reservoir 1618 via the conduit 1665. Thesecond output piston 1640 includes radially extending compensation ports1672 which when moved past a seal 1673 closes off fluid communicationwith the reservoir 1618 via a conduit 1677. The brake pedal unit 1620may be sized and configured so that the compensation ports do not closeoff communication-simultaneously. For example, in the embodiment of thebrake pedal unit 1620, initial movement of the input piston 1634 fromthe input rod 1645 causes movement of the first and second outputpistons 1638 and 1640. The pistons will advance until the compensationports 1664 and 1672 of the input piston 1634 and the second outputpiston 1640, respectively, are closed off by moving past the respectiveseals 1660 and 1673. At this point, the compensation ports 1670 of thefirst output piston 1640 have not yet moved beyond the seal 1671.Additional movement of the input piston 1634 causes the first outputpiston 1638 to advance towards the second output piston 1640. Note thatflow of fluid out of the pressure chamber 1828 of the second outputpiston 1640 is prevented from flowing due to the closed positions of thebase brake valves of the power pack 1614 as described above with respectto operation of the brake system 10 of FIG. 1. The brake pedal unit 1620may be configured such that the first output piston 1638 moves only aslight distance prior to the compensation ports 1670 moving past theseal 1671 such that the pin 1647 is centered in the slot 1643 or atleast located in a spaced relationship from the ends of the slot 1643.Thus, the pin and slot connection permits slight movement between thefirst and second output pistons 1638 and 1640 in each axial direction.Under normal boosted operation, further movement of the input piston1634 causes compression of the springs of the pedal simulator 1621 andthe first and second output pistons do not advance further. However,during a manual push through brake operation the first and second outputpistons 1638 and 1640 may advance rightward pressurizing theirrespective pressure chambers 1798 and 1828. The first and second outputpistons 1638 and 1640 are free to move slightly relative to one anotherto help equalize pressures within the output chambers 1798 and 1828.This performs a similar function as the wobble plate 94 of the brakepedal unit 20 of FIG. 1.

Alternatively, the brake pedal unit 1620 could be configured such thatin the rest mode, the pin 1647 is centered in the slot 1643 and thecompensation ports of the first and second output pistons 1638 and 1640are closed off at the same time. This configuration reduces “lost”travel or movement of the first output piston 1638 when transferring tomanual push through mode.

There is illustrated in FIG. 22 an alternate embodiment of a vehiclebrake system, indicated generally at 1800. Similar to the abovedescribed brake systems, the brake system 1800 may suitably be used on aground vehicle such as an automotive vehicle having four wheels and awheel brake for each wheel. Furthermore, the brake system 1800 can beprovided with other braking functions such as anti-lock braking (ABS),other slip control features, and regenerative braking blending toeffectively brake the vehicle. The brake system 1800 is similar infunction and structure of some aspects of the above described brakesystems. In particular, the brake system 1800 includes a power pack orblock 1802 having similar components and functioning in a similar manneras the components of the block 464 of the brake system 460 describedabove with respect to FIG. 7. Instead, the brake system 1800 includes abrake pedal unit 1820 that is similar in structure and function as thebrake pedal unit 1620 illustrated in FIGS. 20 and 21.

There is illustrated in FIG. 23 an alternate embodiment of a brake pedalunit, indicated generally at 1920. The brake pedal unit 1920 is similarin structure and function as the brake pedal unit 1620 described abovewith the respect to FIG. 21. One of the differences is that the brakepedal unit 1920 includes a travel sensor assembly, indicated generallyat 1930, mounted in a housing 1924 of the brake pedal unit 1920 fordetecting the position of an input piton 1934. The travel sensorassembly 1930 includes an elongated bore 1931 formed in the housing 1924parallel to the axial movement of the input piston 1934 (and otherpistons) of the brake pedal unit 1920. A non-magnetic rod 1933 isslidably disposed in the bore 1931. The rod 1933 is moved or pushed inthe rightward direction by a flange member 1935 attached to a coupler1946 (or directly to the input piston 1934) when the input piston 1934is moved. A return spring 1937 biases the rod 1933 against the flangemember 1935. A magnet 1941 is attached to the end of the rod 1933. Asensor unit, schematically shown at 1943 is mounted on the housing 1924and includes sensor elements which can generate a signal based on theposition of the magnet 1941 relative to the sensor unit 1943. Thus,movement of the input piston 1934 can be detected. This type of sensorarrangement may be more desirable and/or accurate than a position sensormounted on the brake pedal linkage connected to an input rod 1945.

There is illustrated in FIG. 24 an alternate embodiment of a vehiclebrake system, indicated generally at 2000. Similar to the abovedescribed brake systems, the brake system 2000 may suitably be used on aground vehicle such as an automotive vehicle having four wheels and awheel brake for each wheel. Furthermore, the brake system 2000 can beprovided with other braking functions such as anti-lock braking (ABS),other slip control features, and regenerative braking blending toeffectively brake the vehicle. The brake system 2000 is similar infunction and structure as the brake system 1600 illustrated in FIG. 20.One of the differences is that the brake system 2000 is configured for avertically split system such that the wheel brakes associated with thefront wheels are in fluid communication with one circuit, and the wheelbrakes associated with the rear wheels are in fluid communication withanother circuit. For example, as shown in FIG. 24, the wheel brake 2016a may be associated with the left front wheel, and the wheel brake 2016b may be associated with the right front wheel. The wheel brake 2016 cmay be associated with the left rear wheel, and the wheel brake 2016 dmay be associated with the right rear wheel. For this configuration, thebrake pedal unit 2020 may be configured such that the diameter of thesecond output piston 2040 (which is associated with the rear circuit) ismade smaller (as schematically shown in FIG. 24) to increase the volumein the front circuit. The alteration is easily accomplished by theembodiment of the brake pedal unit 2020 compared to the brake pedal unit20 utilizing parallel side by side output pistons.

There is illustrated in FIG. 25 an alternate embodiment of a vehiclebrake system, indicated generally at 2100. Similar to the abovedescribed brake systems, the brake system 2100 may suitably be used on aground vehicle such as an automotive vehicle having four wheels and awheel brake for each wheel. Furthermore, the brake system 2100 can beprovided with other braking functions such as anti-lock braking (ABS),other slip control features, and regenerative braking blending toeffectively brake the vehicle. The brake system 2100 is similar infunction and structure of some aspects of the above described brakesystems, such as for example the brake system 1800 illustrated in FIG.22.

The brake system 2100 generally includes a hydraulic control unit 2112and a block or power pack assembly 2114. The hydraulic control unit 2112includes a brake pedal unit 2120 which may be similar in structure andfunction as the brake pedal unit 20 described above (or any of the otherbrake pedal units described herein). The hydraulic control unit 2112includes a simulation valve 2174 which may be similar in structure andfunction as the simulation valve 74 described above. The power packassembly 2114 may be similar in structure and function as the power packassembly 1802 of FIG. 22.

One of the differences between the brake pedal unit 2120 and the otherbrake pedal units described herein is that the brake pedal unit 2120 hasfirst and second output pistons 2138 and 2140 having a differentmechanical connection therebetween. Similar to the first and secondoutput pistons 1638 and 1640 of FIG. 20, the first and second outputpistons 2138 and 2140 are mechanically connected together such thatthere is limited play or movement between the pistons 2138 and 2140.This type of connection permits the first and second output pistons 2138and 2140 to move relative to one another by relatively small incrementsto compensate for pressure and/or volume differences in their respectiveoutput circuits. The brake pedal unit 2120 includes a locking member2200 having a first end 2202 and a second end 2204. The first end 2202is trapped within a recess 2206 formed in the first output piston 2138.The second end 2204 is trapped within a recess 2208 formed in the secondoutput piston. A first spring 2210 is housed within the recess 2206 ofthe first output piston 2138 and biases the locking member 2200 in adirection towards the first output piston 2138 and away from the secondoutput piston 2140. A second spring 2212 is housed within the recess2208 of the second output piston 2140 and biases the locking member 2200in a direction towards the first output piston 2138 and away from thesecond output piston 2140. The springs 2210 and 2212 and the lockingmember 2200 maintain the first and second output piston at a spacedapart distance from one another while permitting limited movementtowards and away from each other.

There is illustrated in FIG. 26 an alternate embodiment of a brake pedalunit, indicated generally at 2320. The brake pedal unit 2300 is similarin structure and function as the brake pedal units 1620 and 1920 ofFIGS. 21 and 23, respectively. The brake pedal unit 2320 includes apedal simulator, indicated generally at 2321. The pedal simulator 2321includes a simulation chamber 2344 which is in selective fluidcommunication with a reservoir (not shown) via a simulation valve (notshown). The pedal simulator 2320 includes a pair of axially alignednested output pistons 2338 and 2340. The brake pedal unit 2320 includesa housing 2324, an input rod 2345, a coupler 2346, an input piston 2334,a caged spring assembly 2402 having a pin 2404, retainer 2406, a lowrate simulator spring 2408, and a high rate simulator spring 2430. Tocreate a different spring rate characteristic, the pedal simulator 2321may include an additional spring 2431 which may have a different springrate than the springs 2430 and 2408. The springs 2430, 2431, and 2408may be preloaded when the brake pedal unit 2300 is in a rest mode whenpressure is not generated in the pedal simulator 2321 or first andsecond output chambers 2398 and 2428. The springs 2430 and 2431 actagainst one another via a first separator member or sleeve 2433 disposedtherebetween. The sleeve 2433 is slidably mounted on the pin 2404. Thesprings 2430 and 2408 act against one another via a separator member2435.

Similar to the brake pedal unit 1320 of FIG. 17, the input piston 2334of the brake pedal unit 2300 may include one or more frustoconicalsurfaces 2500 which frictionally engage with an O-ring 2502 to providean increased or progressive rate of frictional contact the farther theinput piston 2334 is advanced within the brake pedal unit 2300. Thisprogressive rate of frictional contact can be accomplished by anysuitable manner. There is illustrated in FIG. 26 an alternate embodimentof a friction generated mechanism, indicated generally at 2550. Althoughthe brake pedal unit 2300 is illustrated with both the frictiongenerated mechanism 2550 and the frustoconical surface 2500 and O-ring2502 arrangement, it may be desirable to only include one of thesearrangements.

The friction generated mechanism 2550 includes an elastomeric member orcup 2560 which is mounted within a recess 2562 formed in the firstoutput piston 2338. The cup 2560 includes a bore 2564 having an innerwall 2566. Optionally, the inner wall 2566 includes a plurality ofradially inwardly extending ribs 2568. The ribs 2568 may be formed suchthat they are sloped relative to the axial direction of the movement ofthe output pistons 2338 and 2340 such that the inner diameter ordistance between the plurality of ribs 2568 decreases as movingrightward as viewing FIG. 26. The inner wall 2566 or the ribs 2568define engagement surfaces that contact with a sliding member or washer2570 slidably disposed within the bore 2564 of the cup 2560. The washer2570 is mounted on the end of the pin 2404 and moves therewith. Thewasher 2570 has an outer cylindrical surface 2572 which defines anengagement surface. During operation of the pedal simulator 2550, thewasher 2570 advances into the bore 2564 of the cup 2560. Due to thesloped engagement surfaces of the ribs 2568, the rate of friction isincreased, preferably in a progressive manner, as the input piston 2334moves towards the first output piston 2338. The deformation of the ribs2568 as the washer 2579 slides in the bore 2564 results in the increasedfrictional forces. As discussed above with respect to the brake pedalunit 1320 of FIG. 17, the cross-sectional profile or slopes of the ribs2568 can be configured or shaped to provide a desired progressivehysteresis such that there is increased friction with an increase intravel of the input piston 2334.

Another difference between the brake pedal unit 2300 and the other brakepedal units described herein is the type of mechanical connectionbetween the first and second output pistons 2338 and 2340. The first andsecond output pistons 2338 and 2340 are mechanically connected togethersuch that there is limited play or movement between the pistons 2338 and2340. This type of connection permits the first and second outputpistons 2338 and 2340 to move relative to one another by relativelysmall increments to compensate for pressure and/or volume differences intheir respective output circuits. As described above with respect to thebrake pedal unit 1620 of FIG. 21, it may be desirable under certainfailure modes that the second output piston 2340 is connected to thefirst output piston 2338.

As shown in FIG. 26, the first output piston 2338 includes an extension2580 which is partially received in a recess 2582 formed in the secondoutput piston 2340. The second output piston 2340 includes an axiallyextending slot 2584 in communication with the recess 2582. A pin 2586 ismounted on the extension 2580 in a preferably fixed arrangement andextends through the slot 2584. The brake pedal unit 2300 is shown in itsrest mode in FIG. 26 when pressure is not generated in the pedalsimulator 232, the first output chamber 2398, and the second outputchamber 2428. It is noted that the pin 2586 is centrally located withinthe slot 2584, i.e., the pin 2586 does not abut against either of theends of the slot 2584. The brake pedal unit 2300 further includes aspring 2588 which acts on the right-hand end of the first output piston2338 and against a spring retainer sleeve or washer 2590. The washer2590 is slidably mounted on the extension 2580. The washer 2590 includesan end 2592 which abuts the pin 2586. In the rest mode, the spring 2588pushes the washer against the end of the second output piston 2340. Thewasher 2590 is dimensioned such that the end 2592 of the washer 2590positions the pin 2586 within the central or middle portion of the slot2584. The spring force of the spring 2588 is preferably greater than thespring force of a return spring 2596 which biases the second outputpiston 2340 towards the first output piston 2338.

The principle and mode of operation of this invention have beenexplained and illustrated in its preferred embodiment. However, it mustbe understood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its spiritor scope.

The invention claimed is:
 1. A brake system comprising: a brake pedal; apedal simulator; a brake pedal unit including an input piston connectedto said brake pedal for operating said pedal simulator during a normalbraking mode, said brake pedal unit further including first and secondoutput pistons actuated by said input piston during a manual pushthrough mode such that said first output piston generates brakeactuating pressure at a first output of said brake pedal unit and saidsecond output piston generates actuating pressure at a second output ofsaid brake pedal unit; a hydraulic pressure source for supplying fluidat a controlled boost pressure; and a hydraulic control unit adapted tobe hydraulically connected to said brake pedal unit and said hydraulicpressure source, wherein said hydraulic control unit includes a slipcontrol valve arrangement and a switching base brake valve arrangementfor switching the brake system between the normal braking mode, whereinboost pressure from the hydraulic pressure source is supplied to firstand second vehicle brakes, and the manual push through mode, wherein thebrake actuating pressure from said first and second output pistons issupplied to the first and second vehicle brakes, respectively; whereinsaid first and second output pistons are aligned axially with respect toone another and configured in a nested arrangement such that a portionof one output piston is disposed in a recess of the other output piston.2. The brake system of claim 1, wherein the input piston and said firstand second output pistons are aligned axially with respect to oneanother.
 3. The brake system of claim 1, wherein said first and secondoutput pistons are mechanically connected together.
 4. A brake systemcomprising: a brake pedal; a pedal simulator; a brake pedal unitincluding an input piston connected to said brake pedal for operatingsaid pedal simulator during a normal braking mode, said brake pedal unitfurther including first and second output pistons actuated by said inputpiston during a manual push through mode such that said first outputpiston generates brake actuating pressure at a first output of saidbrake pedal unit and said second output piston generates actuatingpressure at a second output of said brake pedal unit; a hydraulicpressure source for supplying fluid at a controlled boost pressure; anda hydraulic control unit adapted to be hydraulically connected to saidbrake pedal unit and said hydraulic pressure source, wherein saidhydraulic control unit includes a slip control valve arrangement and aswitching base brake valve arrangement for switching the brake systembetween the normal braking mode, wherein boost pressure from thehydraulic pressure source is supplied to first and second vehiclebrakes, and the manual push through mode, wherein the brake actuatingpressure from said first and second output pistons is supplied to thefirst and second vehicle brakes, respectively; wherein said first andsecond output pistons are mechanically connected together to limit themovement of said first output piston relative to said second outputpiston.
 5. The brake system of claim 4, wherein said brake pedal unitincludes a first spring biasing said second output piston axially awayfrom said first output piston.
 6. The brake system of claim 5, whereinsaid brake pedal unit further includes a second spring biasing saidsecond output piston axially towards said first output piston.
 7. Thebrake system of claim 4, wherein a first output chamber is defined by abore formed in a housing of said brake pedal unit, said first outputpiston, and said second output piston, and wherein a second outputchamber is defined by said bore of said housing and said second outputpiston, and wherein said first and second output pistons aremechanically connected together such that an increase in pressure ofsaid first output chamber relative to the pressure within said secondoutput chamber prevents said second output piston from moving furtheraway from said first output piston.
 8. A brake system comprising: abrake pedal; a pedal simulator; a brake pedal unit including an inputpiston connected to said brake pedal for operating said pedal simulatorduring a normal braking mode, said brake pedal unit further includingfirst and second output pistons actuated by said input piston during amanual push through mode such that said first output piston generatesbrake actuating pressure at a first output of said brake pedal unit andsaid second output piston generates actuating pressure at a secondoutput of said brake pedal unit; a hydraulic pressure source forsupplying fluid at a controlled boost pressure; and a hydraulic controlunit adapted to be hydraulically connected to said brake pedal unit andsaid hydraulic pressure source, wherein said hydraulic control unitincludes a slip control valve arrangement and a switching base brakevalve arrangement for switching the brake system between the normalbraking mode, wherein boost pressure from the hydraulic pressure sourceis supplied to first and second vehicle brakes, and the manual pushthrough mode, wherein the brake actuating pressure from said first andsecond output pistons is supplied to the first and second vehiclebrakes, respectively; wherein a first output chamber is defined by abore formed in a housing of said brake pedal unit, an outer cylindricalsurface of said first output piston, and said second output piston suchthat said first output chamber has annular shape surrounding a portionof said second output piston, and wherein a second output chamber havinga generally cylindrical shape is defined by said bore of said housingand an end of said second output piston.
 9. A brake system comprising: abrake pedal; a pedal simulator; a brake pedal unit including an inputpiston connected to said brake pedal for operating said pedal simulatorduring a normal braking mode, said brake pedal unit further includingfirst and second output pistons actuated by said input piston during amanual push through mode such that said first output piston generatesbrake actuating pressure at a first output of said brake pedal unit andsaid second output piston generates actuating pressure at a secondoutput of said brake pedal unit; a hydraulic pressure source forsupplying fluid at a controlled boost pressure; and a hydraulic controlunit adapted to be hydraulically connected to said brake pedal unit andsaid hydraulic pressure source, wherein said hydraulic control unitincludes a slip control valve arrangement and a switching base brakevalve arrangement for switching the brake system between the normalbraking mode, wherein boost pressure from the hydraulic pressure sourceis supplied to first and second vehicle brakes, and the manual pushthrough mode, wherein the brake actuating pressure from said first andsecond output pistons is supplied to the first and second vehiclebrakes, respectively; and further including a sliding member connectedto said input piston and slidably engaging an engagement surface of anelastomeric member during axial movement of said input piston, whereinthe sliding engagement between said sliding member and said engagementsurface provides a progressively increasing rate of frictiontherebetween as the input piston advances in the brake pedal unit, andwherein said elastomeric member is attached to said first output piston.10. The brake system of claim 9, wherein said elastomeric member ismounted in a recess formed in the first output piston.
 11. The brakesystem of claim 9, wherein said elastomeric member includes a borehaving an inner wall defining said engagement surface, and wherein saidsliding member slidably engages said inner wall of said bore.
 12. Thebrake system of claim 11, wherein said inner wall includes a pluralityof radially inwardly extending ribs.
 13. The brake system of claim 12,wherein said plurality of ribs define said engagement surface andinclude surfaces which are sloped relative to the axial direction oftravel of said input piston.
 14. The brake system of claim 13, whereinsaid sliding member includes a cylindrical outer surface which engageswith said plurality of ribs.
 15. A brake system comprising: a brakepedal; a pedal simulator; a brake pedal unit including an input pistonconnected to said brake pedal for operating said pedal simulator duringa normal braking mode, said brake pedal unit further including first andsecond output pistons actuated by said input piston during a manual pushthrough mode such that said first output piston generates brakeactuating pressure at a first output of said brake pedal unit and saidsecond output piston generates actuating pressure at a second output ofsaid brake pedal unit; a hydraulic pressure source for supplying fluidat a controlled boost pressure; and a hydraulic control unit adapted tobe hydraulically connected to said brake pedal unit and said hydraulicpressure source, wherein said hydraulic control unit includes a slipcontrol valve arrangement and a switching base brake valve arrangementfor switching the brake system between the normal braking mode, whereinboost pressure from the hydraulic pressure source is supplied to firstand second vehicle brakes, and the manual push through mode, wherein thebrake actuating pressure from said first and second output pistons issupplied to the first and second vehicle brakes, respectively; whereinsaid pedal simulator includes a caged spring assembly positioned betweensaid input piston and said first output piston, and wherein said cagedspring assembly includes first, second, and third simulator springs eachhaving a different spring rate.
 16. The brake system of claim 15,wherein each of said first, second, and third simulator springs arepreloaded when said brake pedal unit is in a rest mode such thatpressure is not generated in said pedal simulator, said first outputchamber, and said second output chamber.
 17. The brake system of claim16, wherein said first simulator spring acts against said secondsimulator spring via a first separator member, and wherein said secondsimulator spring acts against said third simulator spring via a secondseparator member.